###Syllabification side point

This is a side point, but one thing to keep in mind is that even with a short i sound, "decode" would not be pronounced like "Dick-oh-d". You may have heard a rule about "short" vowel sounds being grouped with a following consonant, but that rule only applies when in syllables that have some degree of stress. A short vowel sound can occur at the end of a fully unstressed syllable (the exact circumstances in which you find short vowels are different in different accents).

When the syllable "de-" is pronounced with a short i sound, it is fully unstressed, and so the following consonant acts as the start of the second syllable in the word, not as the end of the first syllable. If a pronunciation of decode with the short i sound in the first syllable exists (I'm not sure about that), it would have to be "dih-kohd" [dɪˈkʰoʊd].

One more thing: in some accents (in particular, very often in North American English), the short i sound in unstressed syllables is not clearly distinct from the schwa sound. This is called the "weak vowel merger".

###"Demilitarize" with a short vowel is in MW; "decode" seems less likely to me to have a short vowel Merriam Webster corroborates the existence of two pronunciations for demilitarize: one where the first syllable has the long e sound and potentially has secondary stress, and one where the first syllable has the short i sound and is unstressed.

For decode and demilitarize, it certainly wouldn't be an issue to always pronounce the first syllable with a long e sound. But there are some words starting with de- where that would be unusual. The verbs deposit, derive, deceive, delude all start with a syllable that is etymologically derived from the Latin prefix de-, but the second parts of these verbs are not used as independent elements in English. Because of this, they tend not to be pronounced with a long e sound in the first syllable.

One way to think of it is that as a productive English prefix, de- is pronounced with a long e sound. So when de- has been used in English to form a new English word from another English word, it is pronounced with a long e sound. But in words where the prefix was added before the word entered English, it may be pronounced in other ways (when unstressed, as [dɪ] or [də]; when stressed, as [dɛ], as in demonstrate, definition, designation).

###Syllabification side point

This is a side point, but one thing to keep in mind is that even with a short i sound, "decode" would not be pronounced like "Dick-oh-d". You may have heard a rule about "short" vowel sounds being grouped with a following consonant, but that rule only applies when in syllables that have some degree of stress. A short vowel sound can occur at the end of a fully unstressed syllable (the exact circumstances in which you find short vowels are different in different accents).

When the syllable "de-" is pronounced with a short i sound, it is fully unstressed, and so the following consonant acts as the start of the second syllable in the word, not as the end of the first syllable. If a pronunciation of decode with the short i sound in the first syllable exists (I'm not sure about that), it would have to be "dih-kohd" [dɪˈkʰoʊd].

One more thing: in some accents (in particular, very often in North American English), the short i sound in unstressed syllables is not clearly distinct from the schwa sound. This is called the "weak vowel merger".

###"Demilitarize" with a short vowel is in MW; "decode" seems less likely to me to have a short vowel Merriam Webster corroborates the existence of two pronunciations for demilitarize: one where the first syllable has the long e sound and potentially has secondary stress, and one where the first syllable has the short i sound and is unstressed.

For decode and demilitarize, it certainly wouldn't be an issue to always pronounce the first syllable with a long e sound. But there are some words starting with de- where that would be unusual. The verbs deposit, derive, deceive, delude all start with a syllable that is etymologically derived from the Latin prefix de-, but the second parts of these verbs are not used as independent elements in English. Because of this, they tend not to be pronounced with a long e sound in the first syllable.

One way to think of it is that as a productive English prefix, de- is pronounced with a long e sound. So when de- has been used in English to form a new English word from another English word, it is pronounced with a long e sound. But in words where the prefix was added before the word entered English, it may be pronounced in other ways (when unstressed, as [dɪ] or [də]; when stressed, as [dɛ], as in demonstrate, definition, designation).

###Sometimes context or emphasis can affect the pronunciation of a prefix like de-

Here is an interesting article that talks about some examples, like defuse and descend:

"Pronunciation of Prefixed Words in Speech: The Importance of Semantic and Intersubjective Parameters", by Nicolas Videau and Sylvie Hanote, Lexis [Online], 9 | 2015, Online since 13 May 2015

This is a side point, but one thing to keep in mind is that even with a short i sound, "decode" would not be pronounced like "Dick-oh-d". You may have heard a rule about "short" vowel sounds being grouped with a following consonant, but that rule only applies when in syllables that have some degree of stress. A short vowel sound can occur at the end of a fully unstressed syllable (the exact circumstances in which you find short vowels are different in different accents).

When the syllable "de-" is pronounced with a short i sound, it is fully unstressed, and so the following consonant acts as the start of the second syllable in the word, not as the end of the first syllable. If a pronunciation of decode with the short i sound in the first syllable exists (I'm not sure about that), it would have to be "dih-kohd" [dɪˈkʰoʊd].

One more thing: in some accents (in particular, very often in North American English), the short i sound in unstressed syllables is not clearly distinct from the schwa sound. This is called the "weak vowel merger".

Merriam Webster corroborates the existence of two pronunciations for demilitarize: one where the first syllable has the long e sound and potentially has secondary stress, and one where the first syllable has the short i sound and is unstressed.

For decode and demilitarize, it certainly wouldn't be an issue to always pronounce the first syllable with a long e sound. But there are some words starting with de- where that would be unusual. The verbs deposit, derive, deceive, delude all start with a syllable that is etymologically derived from the Latin prefix de-, but the second parts of these verbs are not used as independent elements in English. Because of this, they tend not to be pronounced with a long e sound in the first syllable.

###Syllabification side point

This is a side point, but one thing to keep in mind is that even with a short i sound, "decode" would not be pronounced like "Dick-oh-d". You may have heard a rule about "short" vowel sounds being grouped with a following consonant, but that rule only applies when in syllables that have some degree of stress. A short vowel sound can occur at the end of a fully unstressed syllable (the exact circumstances in which you find short vowels are different in different accents).

When the syllable "de-" is pronounced with a short i sound, it is fully unstressed, and so the following consonant acts as the start of the second syllable in the word, not as the end of the first syllable. If a pronunciation of decode with the short i sound in the first syllable exists (I'm not sure about that), it would have to be "dih-kohd" [dɪˈkʰoʊd].

One more thing: in some accents (in particular, very often in North American English), the short i sound in unstressed syllables is not clearly distinct from the schwa sound. This is called the "weak vowel merger".

###"Demilitarize" with a short vowel is in MW; "decode" seems less likely to me to have a short vowel Merriam Webster corroborates the existence of two pronunciations for demilitarize: one where the first syllable has the long e sound and potentially has secondary stress, and one where the first syllable has the short i sound and is unstressed.

For decode and demilitarize, it certainly wouldn't be an issue to always pronounce the first syllable with a long e sound. But there are some words starting with de- where that would be unusual. The verbs deposit, derive, deceive, delude all start with a syllable that is etymologically derived from the Latin prefix de-, but the second parts of these verbs are not used as independent elements in English. Because of this, they tend not to be pronounced with a long e sound in the first syllable.

One way to think of it is that as a productive English prefix, de- is pronounced with a long e sound. So when de- has been used in English to form a new English word from another English word, it is pronounced with a long e sound. But in words where the prefix was added before the word entered English, it may be pronounced in other ways (when unstressed, as [dɪ] or [də]; when stressed, as [dɛ], as in demonstrate, definition, designation).