But if Jellyfin isn't a registered 501(c) charity, then the point is moot.

There are many different options to register jellyfin in the US, and many other options to register in other countries. Each as their own set of rules. If Jellyfin isn't registered in any way then the law gets even more complex (if you are registered anywhere other countries will generally recognize that and accept the laws of where you registered apply, but if not registered nobody knows but the lawyers can spend millions fighting it out)

If it's able to be treated as a gift, it is not taxable to the recipient. I an not a tax professional, but I would expect most of what people call "donations" to Jellyfin would be considered gifts by the tax court.