What was the way of life like such that ancient Greek philosophers (i.e Socrates 5th century BC) to more “modern” 16–18th century philosophers could do just that – philosophy – and survive? What changed culturally that this is no longer possible?

More can always be said on the matter, especially about what the later-era philosophers did to support themselves, so if anyone who knows about them would like to expand on how they did it, please feel free to do so!

For the meantime, OP, we have some previous posts on the matter that focus on the Greeks, which should make for some interesting reading.

u/toldinstone covers some specific philosophers, focusing particularly on Sokrates, in fact;
and u/Iphikrates looks at how the Greek leisure class as a whole, from which the philosophers arose, were supported.