What’s the state of third-party clients for GitHub, Reddit, and YouTube?
Apollo is a Reddit client for iOS and the major feature it doesn't have from the official client is "Chat". To be clear, you can still send/receive DMs, but the IM chat is not available. For time sensitive events (/r/place) I believe there was support in the official app, but that makes sense not to expose to 3rd parties.
Most other "features" can be recreated by the client independently as long as they have access to your posts/history.
All of that said, I would not hold my breath that this will still be true in 10 years. The only reason that the 3rd party clients are still supported is because Reddit is able to make enough revenue from those users to justify the expense. If/when that changes, so will API access. To their credit, it is possible to spend Reddit coins and give awards via the current API, but you need to use an official client to top up your account. This at least provides a path for 3rd party client users to generate revenue for Reddit directly.
I personally prefer Narwhal on iOS, but if I were forced to use native Reddit app—or really anything resembling ‘new’ Reddit—I’d immediately leave and never come back.
If Apollo stops working, then I assume the other 3rd party apps would also be disabled so then I’m out. I’m a bit too young to have been active during RSS and and IRC days, but I’m exploring what a modern version of that experience looks like in case Reddit goes kaput.
A much easier way for a first party to kill third parties that may be hurting their businesses is buying them out and/or incorporating their functionality into the first party apps.
If users are choosing third party apps it's usually because (a) they are breaking some pretty obvious rules, like copyright and/or not displaying ads, in which case they will be in a constant struggle to survive, (b) they are providing functionality the first party does not or cannot provide, in which case it's still beneficial for the first party since the third party app is helping them keep the users.
Youtube is a lot more adversarial because of all the copyright issues, but there's NewPipe, and yt-dlp as well as all the authorized third party clients like non-Android TVs.