Do "elision" and "ratatouille" have unmarked plural forms?
According to Microsoft® Encarta® 2009, the word elision has an unmarked plural elision (no -s suffix) as an alternative to elisions.
Can "elision" be used as a plural form? If so, is it due to its Latin origin?
A similar example I've found is ratatouille, but I am ignorant of the pronunciation of its plural in French.
I can't imagine why Encarta would say that. Merriam-Webster, American Heritage, and the OED fail to mention an alternative plural without the -s. (The same is true for ratatouille. If you want to see for yourself what unusual plurals are indicated, search any of those dictionaries for "octopus" or "deer".)
There is nothing of the Latin that would indicate "elision" can be plural. The Latin word elisio is your run-of-the-mill third declension noun. The plural (nominative and accusative) in Latin is elisiones, which in English is virtually always a simple -s. Cf. L. machinationes with E. "machinations."
My guess is that Encarta is simply wrong or that there might be some specialized, non-standard usage that somehow adopted a singular word for its plural, but I wouldn't trust Encarta to relay that without attestation. You're safer going with the three other dictionaries instead.
Ratatouille, at least in French, has both a singular form (ratatouille) and a plural one (ratatouilles, and only this one).
The plural form is not used much, but makes sense in some context (e.g. toutes les ratatouilles ne se ressemblent pas - ratatouilles are not made equal)
I googled (in quotes)
"many elision"
and found nothing other than a typo:
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwjzo4nWhLvyAhVCK80KHb9IA5AQFnoECAcQAQ&url=https%3A%2F%2Fartistryalliance.net%2Fwp-content%2Fuploads%2F2019%2F05%2Fconfigurations2.pdf&usg=AOvVaw2xICOW1DOYVnrjBIdWJBQD "How many elision can you find in The Orill Wheel"
Right on p. 1 the document mentions "elisions" in all caps and in bold. At the bottom, you see "Now find elisions in..." and "How many other elisions..." It's a typo.