Which type of followers still benefit from Garrison mission XP?

Solution 1:

Once a follower reaches level 100, any experience they gain from there goes into increasing their quality (green → blue → purple).

In-game, you can still see the XP bar on non-epic (green/blue) level 100 followers. See for example Songla and Hulda in your screenshot, which would have a bar like this:

Epic (purple) followers do not have such a bar anymore: they do not benefit from base or bonus XP at all. For example, your epic character "Ilona" will look like this:

The green "+32 XP" in your screenshot is extra XP a character would get for being overqualified for a quest, but again: since your follower is already epic this XP will go to waste.

Higher quality followers have additional traits and abilities (which let them counter more stuff).

Solution 2:

I send my 100 level characters on XP missions just to clear out my available mission queue; rather than letting them all expire which seems to take longer. I first do any armor upgrade missions, then resources (with Ben Gibb), then gold, then all the XP missions that are short duration (so I have followers available when the other missions are ready). Chance of success is irrelevant so I just send anyone. Not sure this is smart but keeps me busy.