There has been no signs whatsoever that T10 plans to throw in the towel. Quite on the contrary they mentioned the second half of the year in their latest blogpost:
"While we are working through our plans for the second half of the year, we hope you have fun playing the updates we’re working on for the next few months..."
Also, remember that T10 is the main developer of the Forza Tech engine and without it both Forza Horizon and Fable would be without a game engine.
As long as Playstation have Gran Turismo I'm sure that Microsoft will want to have their counterpart in the form of Forza Motorsport.
I'm not sure whether or not it is the cause for the slow down in added tracks, but according to Playground Games themselves both them and T10 are giving assistance to Panic Button with the port to PS5:
"The PlayStation 5 version of Forza Horizon 5, developed by Panic Button in partnership with Turn 10 Studios and Playground Games...". (From the article about the PS5 port on Forza.net)
Lastly for those individuals that are only here to spread doom and gloom I highly recommend that you hang out on the official Forza Forums instead. People love preaching about the downfall of the franchise over there 😂