triple negative...

Firefox's default en-US dictionary (https://searchfox.org/firefox-main/source/extensions/spel...) appears to be based on SCOWL with size 60, where:

> The normal (non-large) dictionaries correspond to SCOWL size 60 and, to encourage consistent spelling, generally only include one spelling variant for a word. The large dictionaries correspond to SCOWL size 70 and may include multiple spelling for a word when both variants are considered almost equal. The larger dictionaries however (1) have not been as carefully checked for errors as the normal dictionaries and thus may contain misspelled or invalid words; and (2) contain uncommon, yet valid, words that might cause problems as they are likely to be misspellings of more common words (for example, "ort" and "calender").
(http://wordlist.aspell.net/hunspell-readme/)

i.e. it's choosing to err on the side of rejecting some correct words, rather than accepting more incorrect words, which seems sensible for a spellchecker. (The user can always ignore the checker if they think they know better, or install a different dictionary.)

In this case, the Google Books 1980-2008 dataset says "aint" has a frequency of 0.0049 per million, which scores 1 out of 5 on the "should include" scale, and is several times less common than non-words like "anit" and "ainst" (http://app.aspell.net/lookup-freq?words=aint).

https://www.thefreedictionary.com/aint suggests some "references in classical literature", but I compared a few against early publications on archive.org: some appear to be OCR errors (the printed version says "ain't"), some appear to be typos (they use "ain't" more often than "aint" in the same edition, and later editions tend to regularise on "ain't"), and some are using deliberate misspelling to depict characters who can't speak or write properly, so genuine uses are even rarer.

Given that evidence, I think aint ain't correct.