I decided to play despit the glitch today.
Thought I'd update people on the results so far.
As of a couple of days ago, I was level 14, and I noticed I wasn't getting XP anymore when trying to do Broken Flowers in the main quest line. (did the whole Broken Flowers as a test, got nothing, reloaded to an earlier save, tried a couple of contracts, got nothing again)
When I started playing today, I was afraid to jump into the game, so at first I ran around and played Gwent (finally developed a half-decent Northern Kingdoms deck in the process), then did some "?" tasks, and finally a couple of level 7 quests that were gray anyway. Got maybe 300 xp that way.
Then I got bored with messing around, and figured I'd roll the dice after all - so I went off and did Destination: Skellige. It worked, I got XP for it. Then I picked up a contract in Skellige, and that worked too. Went from barely level 14 to halfway between 15 and 16. I'm currently in the middle of another main plot quest in Skellige, it remains to be seen how that goes.
Bottom line, encountering the bug doesn't *completely* stop you from gaining quest XP again. (though obviously, this was not a long term test, so it's hard to say anything definite - I don't know how long my good luck will last) However, based on whats some people have said in this thread, I'm not sure it's possible to predict which quests will give XP and which won't. I'm pretty sure I saw someone complaining that they went to Skellige and couldn't find anything that gave XP. Perhaps that's why CDPR have been so close-mouthed as to what exactly the problem is?
I do have a feeling, however, that the game is so filled with redundant ways of getting XP, and has so many quests, that on any given playthrough a large percentage of quests would become "greyed-out" anyway - which is why they're confident in saying this glitch doesn't significantly affect balance. There's probably no way to get 100% of the theoretical quest XP even when the game is working as intended. (which really pisses some people off, so they devs don't want to talk about it)