kapusta: prof. google says—“Inevitable” comes from the Latin word “inevitabilis” – in (not) + evitabilis (avoidable).
“Evitable” never really made it as a regular word in the language. However, I’d say you can use it without any sort of confusion – it’s obviously the opposite of “inevitable”.
A similar unused word is the opposite of “ruthless” – what does it mean to be “ruthful”?
Reuthe was an old word meaning “compassion” or “pity” – today we say “compasionate” rather than “ruthful”.