It depends on the youtuber of course, a few of them have an audience that legitimately allows for a plan b.

For most of them there isn't. They might have seperate portals which they earn money from, but the business still goes poof fast if they get banned from youtube. Without publishing on youtube they won't reach enough of an audience to have a sustainable business.

Twitch alleviates this somewhat for some youtubers. But we don't have to go far back in time to consider an era before that existed.

Not sure I agree with this. Beyond a certain level, subscriptions to a given creator are to that creator, not via the platform. It's true that a lot of consumers won't take the effort to follow the brand off platform, but that's not always the case. I think the recent development of the influencer marketing industry as a whole is proof of this.

Of course, how you ever get remotely famous or established in the first place if the content you create is remotely vulnerable to false copyright claims is another question entirely...

Patreon and similar platforms make your income stream independent of YouTube and give you a second channel of communication to your most dedicated fans that also allows new content even while your YouTube channel is down.