Everything rendered by the server has this potential as it must be deserialized by the browser.
No, but I've definitely seen bespoke, hand-rolled frontend code do it. You can't select SPA frameworks as representative of frontend rendering but look only at hand-rolled PHP from 2002 for backend rendering.
No major SSR framework is XSS-vulnerable by default. Whether you're using Rails, Laravel, Spring, ASP.NET, Phoenix, or whatever, every template engine escapes your strings unless you opt-in with something like Rails's `html_safe`, and they have for over a decade. Unless you've built your own SSR framework by hand with raw string interpolation, the "easy" way to do things is also the right way, just as it is in React.
Yes, congratulations, you pointed out that you can hang yourself either way. That wasn't my point and I acknowledged as much in the first post.
One has an unsanitized escape free method and the other does not.