Moonlight is an unofficial GameStream client so yes if Nvidia completely drops support for GameStream on existing GPUs then Moonlight will stop working.

Nvidia has a proprietary local game streaming function built in to their GPU and the drivers for it. There are other alternatives like Steam's game streaming functionality and the Parsec app, which work with any GPU. In my testing, Nvidia's streaming was the best and that's what I built my streaming setup around.

They've been prioritizing their GeForce Now cloud service over local GameStream for a long time at this point, so this isn't a huge surprise. But for me at least, literally the only reason I bought overpriced Nvidia GPUs for the past few years was because their local network game streaming was much better than any other alternative. This will directly lead to me switching to AMD instead of buying any of the 40 series Nvidia cards.