I can only find one in 2e(Gorellik), who had a slow decline. It doesn't say if that's because of a slow loss of worshippers or not. I know I read about more and some were quicker, but heck if I can remember specifically who.
Belief isn't passive at all. Gods gain followers because they(or their followers) actively recruit. I don't see a difference between cultivating active belief by Iuz and cultivating active belief in some plane to shift it.
Belief is passive in the sense that the god isn't actively going around doing things to get people to worship it. Sure, it grants cleric spells (unless it's getting some other god to grant them, or the cleric is gaining spells through sheer belief), but it's not actually putting out much effort. Its clerics are the ones who actually do the preaching and conversion, while the god sits back and reaps the rewards, like in a pyramid scheme.
The Harmonium put out the active effort to remove chaos from Arcadia, but did so without kindness or decency and often by using cruel methods. The evil of their actions removed enough good to tip the balance from Lawful Neutral Good to just Lawful Neutral.
As for Yeenoghu, he actively instructed his followers to murder any gnoll who didn't worship him. I'm not even 100% sure he allowed for conversion. Some gnolls worshiped Gorellik or even Vaprak and got killed for it. Vaprak wasn't weakened because he had ogres and trolls, but nobody worshiped Gorellik but gnolls. Hence the decline.
However, it's important to realize that, as far as I can tell, Gorellik has been in slow decline since 1e (at least since Yeenoghu was created) but is still alive. So, I don't know how much time has passed in-setting since then (since that, of course, depends on the setting and individual DM), but it's implied that this happened a long time ago, not recently.
Maybe there are a few holdouts who still worship him, on worlds yet untouched by 5e's retcon of gnoll origins. Or maybe he has no worshipers at all but is still holding on due to plot armor, because "god in decline" is potentially more interesting and useful than "god-corpse that nobody knows who it was or who worshiped it."
I think if we're trying to figure out quantities of something, looking at the edition that did it or tried to do it is helpful.
Yeah, but not if it keeps contradicting other pieces of canon. Like with Iuz. He "should" be a lesser god at least. But he's not. Because of plot armor.
"Mortal reverence and worship turned universal ideas into deities. It wasn't until mortals trusted the gods to act on their behalf, in response to sacrifices and prayers, that the gods took shape from the dream-substance of Nyx. Worship remains crucial to the power of the gods, though mortals are generally not aware of their influence. Were a god to cease being worshiped, their might would dwindle."
It's been a while since I read the book. Does it say what
dwindle means? I
imagine it would mean that they would lose their shape and go back to being dreams and nothing more. However, I don't think that Realms or Greyhawk gods work like that. They don't seem to "dwindle" in that manner. They remain gods, albeit of a lower rank. They don't even have the cool option of becoming a different type of power: