Not many.
Some of the reports from the antitrust trial showed that being on Game Pass cannibalizes sales numbers, and if I were a developer I'd probably be worried about how that hurts the initial spike and long tail of my game. Game Pass seems like it trains consumers into not spending money on games, for reasons we can see in the posts upthread, and it seems like it wouldn't be in the developer's best business interest to do that for the smallest platform by market share when they could just release on Steam and Playstation and Switch instead where people are more willing to buy games.
This is what they've been doing, to a degree. However, their internal practices for releasing games have degraded a bunch since the Xbox 360 days (iirc, one of the original Gears of War producers said on Youtube that Xbox management moved to a more patronage-based system than a results-based one around the time Phil Spencer took over in the Xbox One era, but I'm having trouble finding the video) so they've been trying to get the development muscle by buying out studios and
paying millions to hundreds of millions to get games on Game Pass, per leaks from last year.
This has been a double-edged sword because it's increased the operating cost of the Xbox division to no longer be a negligible part of Microsoft's total revenue. That Activision buyout ate up a huge chunk of Microsoft's cash on hand at a time when interest rates were rising and it's looking like the money people at Microsoft are now expecting a stronger ROI for all that cash they've invested, and that means putting their games on as many platforms as possible and cutting staff to boost margins.
I don't know that it makes sense for developers to put their games on Game Pass when it hurts their sales the way it does and with the way that Xbox is a shrinking platform. This is doubly so when we consider that
the Game Pass deals have dried up, as we would expect to see from the cost cutting measures described above. 70 billion dollars is a lot of money to spend, even for a large corporation like Microsoft, and they can't just keep spending hundreds of millions of dollars on exclusivity deals when they're trying to make back that 70 billion dollar hole in the balance sheet.
However, even setting all that aside, the other elephant in the room is that
developers just can't get their games onto Xbox because the back end is breaking, for reasons related to the aforementioned staff cuts. The publishing end of Xbox is a skeleton crew right now and that's making it difficult for even the people who want to get their games onto Xbox to do so.
So I guess to answer the question at the top of the thread: yes I think Game Pass will kill the Xbox developer community, inasmuch as the things Xbox is doing to try and push Game Pass is hurting the division as a whole in the long-term.