There won’t be UBI because the surplus population will be culled, per Agenda30. There will be slaves who will be allowed to live in corporate/government owned (think fascism) shelters, be issued one set of clothing, and given bugs to eat, but I wouldn’t call those things income by any means.
I'm worried about that too, but I would hope that the Powers that be look at the history books and realize that actively killing billions is bad PR.
If they want to cull the population, I'd simply made conceiving much more difficult. Think: A covid-like virus that doesn't kill but makes conception much less likely (like 99.99% less likely).