In the sentence:
Major refreshes to the OS come about every six months, with the most recent being the May 2019 Update.
What is the use of "being" grammatically? Can any words be added between "recent" and "being"?
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"being" is the verb(-ing) of your 2nd clause. You can separate the sentence into 2 as follows:
Major refreshes to the OS come about every six months. The most recent one is the May 2019 Update.
You can add a noun (mainly the subject) between "recent" and "being". For example:
..., with the most recent refresh being the May 2019 update
However, that is not recommended as it repeats the word "refresh".
As @JohnZhau has correctly stated, "being" in this context is a verb, an "action word", the present participle conjugation of the irregularly conjugating verb "to be", which means "to exist", especially in a specified form, state, or location.
Note that "being" can also a noun in other contexts, meaning one's existence or nature, as well living things (e.g. human beings, sentient beings).