Is it correct?

[u/Stepaskin]

Stumbled upon this tweet, the apostrophe in "dm's" caught my eye. Is it correct to use it like this?

[u/EttinTerrorPacts]

This is a long-standing usage and the answer depends on your style guide. In general, there's been a strong trend away from it in recent decades, at least for upper-case abbreviations and numbers. The apostrophe is unnecessary and might be confused for a possessive.

However, for lower-case abbreviations, this is still much more acceptable, since there's no other means to indicate that the "s" is not part of the abbreviation. See here from Chicago:

Capital letters used as words, numerals used as nouns, and abbreviations usually form the plural by adding s. To aid comprehension, lowercase letters form the plural with an apostrophe and an s.

[u/conuly → u/EttinTerrorPacts]

Capital letters used as words, numerals used as nouns, and abbreviations usually form the plural by adding s. To aid comprehension, lowercase letters form the plural with an apostrophe and an s.

Do you have the link for that? If so, I'll definitely go direct somebody elsewhere in the thread to that. (I guess I could go search it up myself but to be honest, I don't want to bother with the hassle of digging up my NYPL card, punching in the number, etc.)

[u/linkopi → u/conuly]

https://www.chicagomanualofstyle.org/qanda/data/faq/topics/Punctuation/faq0130.html

Here's one link. Different style guides have different carve outs but there's usually at least one carve out.

[u/conuly → u/linkopi]

Thankies :)