Since the live update, none of Third Boosters seem to be working, including the new ones that were added in patch 1.18. I'm guessing this is a mistake, but maybe it was on purpose? Don't know how it happened unless it was to do with the booster reset.
Buddy Boosters do still work though, but only if you select them as a slot booster.
I found this out while trying to only use the Tau, I threw on Chloe's M9 with her default skin. For the first few matches I figured I was lonewolfing, but noticed later on it wasn't showing the booster at all. Tested them in custom games, and can confirm Drake's, Elena's, Sully's, Chloe's, Marlowe's and Eddy's loadouts no longer work. They are all fine in LAN mode though.
Btw, dat Tau is so awkward now, please put it's aim-assist back to how it was. Keep the over-shoulder aiming if you must, but the extra aim-assist has to go, it's only useful for getting me killed. I can post videos of why the current Tau feels (and looks) broken if I must.
I haven't checked all of new combos, but Eddy's doesn't work along with all the original ones, so I think it's safe to say they are all broken.
It's a shame though, before heroes had a huge advantage, 1.18 fixed this, now they have all gone. If it was intentional... I guess it was never going to be completely balanced, but there's still bigger problems than this, and this trick was pretty fun.