As Artur said, I think the discussion IronWolf is remembering was from when I moved Anna to NPC status, and we discussed if she still cut into the other PC's share of encounter XP until I could get her to a narrative point where I could write her out. I'm not sure there was any official vote, but the general consensus as I recall it (I'll try to dig back later to find it) was that if the character helps, the xp and gp should be impacted.
Open/Closed NPC: YES
Open/Closed Backgrounds: YES
I think the question of whether an NPC actually earns xp / gp is kind of a non-issue to me, unless you want to take the extra step of statting out / tracking leveling for all NPCs, which seems like kind of a nightmare.
Since Galen Parsons is becoming our base example (which makes me giggle every time it happens. Hooray improv character making good!), consider: I never came close to statting up a full NPC for him, so he didn't have any official classes, feats, or even skills beyond Elvish as a language.
Qik used Parsons in a combat scenario during "Fury Steps In," which meant at some point he sat down and came up with what he thought were reasonable stats for him given the adventure / CR he'd be involved in, but certainly he didn't apply any real XP or GP to him. He couldn't have, because none existed. And from my way of thinking, he certainly shouldn't have to account for 'where' Parsons gained his levels. As an NPC, we assume stuff happens to him outside adventures that covers this.
The only difference between Parsons and, say, Pari, is that Pari at one point had to track his advancement. We know where he stopped, so I think there's probably a tendency to want to keep comparing to that, but I think that's probably a mistake. The pre-existing character's sheet is certainly a good place to start, but beyond that, I think GM's using open retired-to-NPC characters (or their own closed ones) should be as free to stat up as Qik was, and for the same reasons. Since we have no real "un-retire" mechanic, I guess I don't see the potential for abuse there. Is using your retired character as a regular NPC substantially different from using the same from-scratch NPCs regularly (goodness knows I've gone back to the Gabbiano tree often enough)?
Given all of the above, from my perspective the only question is whether an NPC who participates in encounters reduces the available rewards and by how much. If they gain a unique magical item, it seems a good idea to note it, but otherwise tracking those rewards to a given NPC just seems like a lot of extra bookkeeping that doesn't serve a good purpose.