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Linguistics only follows what people use and accept over time. Voluntaryism (with the Y) has a specific meaning: https://en.m.wikipedia.org/wiki/Voluntaryism

Without the Y means something else: https://en.m.wikipedia.org/wiki/Voluntarism

Yeah, I had a long discussion about this on fb recently. I prefer Voluntaryist.

Adding Suffixes
Adding consonant suffixes is easy. Just add them, but if a word ends with a y, you must change y to an i before adding any suffix! Common suffixes include: -ness, -less, -ly, -ful, -hood, -wise, -cess, -ment, -ty, -ry, -ward, -age, -ant, -ance, -al, -ism, -able, -an, -es, -ed, -er, -est, -y, -ist, -ish, -ing, -ar, -on, -ous, -or, -ual, -unt, -um, -us, and -ive.

https://www.headsupdyslexia.com/rules-to-our-language.html

"You must!" sounds pretty strict when in reality language continually adjusts based on common meaning and shared usage. The purpose of language is communication. Using a word that means something other than what you intend because it's more grammatically correct is not good communication.

That said, I appreciate your passion for language. English is weird.

Regardless, it’s a side argument...not to detract from the point/purpose of your post.

The link provided even synonymizes the words. It’s a matter of preference, ultimately. I like “voluntaryist” because it seems to be more precisely (accurately) defined.