Before you close this as "feature request", Grum asked for a ticket so he doesn't forget. Also, Jeb previously considered it and suggested how it would be fixed.

The issue: mapmakers and server owners cannot disable experience received from trading villagers. This drastically breaks the balance of some maps, as shown in the attached GIF.

The fix, proposed by Jeb, is to add a per-offer NBT tag determining whether or not that offer can give the player experience. This would allow mapmakers and server owners to manually disable trading XP from custom villagers.

Again, I'm only posting this because Grum wanted to know if it has a ticket, and didn't want to forget about it. I don't think closing this will help, but I suppose Grum will re-open it if he wants it to stay open.