I've read the message:
Kindly let know about the pay per product descripton
I'm in doubt if it is ok to use let know without a pronoun like let me know,does this sound natural or can be used in any context?
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"Let know" is not idiomatic. You are correct that you do need to use some subject, but it doesn't have to be a pronoun.
Let grandma know her favorite TV show is on.
Let the cat know it's time for dinner.
Let all the good little children, in every country, all over the world know that Santa is coming.
You can't use the phrase "let know" without a direct object. You have to say "let us know", "let them know", "let me know", "let the supervisor know", etc - not just "let know" on its own. It is transitive.
It is rare (and almost always unidiomatic) to use "let" in the passive. You can say "She let me try", but you can't say "I was let try (by her)".
Similarly, you can say "She let me know", but you should avoid "I was let know".
Interestingly, "let go" works in the passive, at least colloquially. "He was let go" means "he was dismissed from employment".