It's been proven on reddit and the forums about 50 times now, the couple quests you don't get XP from if you choose to "max" XP you can do side quests that would usually turn grey and get the same amount of XP. They created more side quests than you can do in the allowed levels...
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You guys are missing the core of the game it's THE STORY, do you get to choose between classes? NOPE so it's not a "get to the highest level possible" RPG. With or without the missing XP whether it's grey quests, outleveling or missing XP you can STILL play through the story and get maxed out for the content that's provided and play all the quests in the game.... SO many games are beat to bones in a week by AAA studios, it's only later when they add more content, modes or mods that you can play the lets level till I get bored game.
The same reddit and forums you are talking about also indicate plenty of players that do not receive XP for more than "a couple" of quests. In fact, many players have reported that they did not receive XP from quests for weeks now, having the main quests out level them. If CDPR is confident the impact will be low, they need to quantify the impact, so players can make their own decisions.
Right now, players DON'T KNOW if can can beat the game properly with this defect, cause no one officially gave them a quantifiable answer.