I can only speak for French. It’s a whole letter representing a vowel called “l’e dans l’o” (the e in the o). It’s completely different from an accent and doesn’t work according to the same rules as accents. I know it may sound crazy but yes, there are rules/a history for diacritics in French, and that letter is not part of it,

It does make two different sounds depending on the word though, so maybe œ should get an accent?!