Spoiler: Do not read further if you have not yet played the March 23 puzzle.

SPECS is not a word. It is an abbreviation of specifications.  Or spectacles.

are there rules eliminating abbreviations somewhere?

Abbreviations have a period at the end, as abbr. Specs is a truncated form of spectacles which, by common usage, is also part of the American lexicon at least.

it is in the dictionary as plural informal of spectacles

Agree completely. Abbrevs are a copout.

What about "phone" (short for telephone) or "auto" (short for automobile) or "flu" (short for influenza) - do you consider these abbreviations instead of words?   English is an evolving language with new words added regularly.  Often, yesterday's abbreviation becomes today's word.    All word games need some sort of definitive reference for what is considered a valid word - seems like the Dordle folks are using Wiktionary as their source.  If it's in Wiktionary, then it's a valid word. 
The game tells you if the letter combo is considered valid - I've learned a couple new words that way :)