There are a number of prominent YouTubers who advertise their own alternative platforms fairly regularly, if "indirectly". There are also videos about LTT's own video service Floatplane on YouTube [0]
I'm guessing that either there's some clause in the YouTube partner/adsense program saying you can't just redirect your viewers elsewhere or you can't upload videos with the sole purpose of saying "go here for more videos".
That said, I'd be interested to see the details of what GP was talking about, because I believe it exists, I just wonder what the exacts are.
[0] https://youtu.be/oOOOfZWXPu4