I'm not sure when it changed (sometime this year?) but GH automatically changes the PR base now.
I do stacked PRs for complex changes, and Github has been updating branch bases to support this workflow for at least two years now.
There are a few free open source tools available these days to manage the stacked PRs. Eg: https://github.com/aviator-co/av
I do wish Gitlab would spend some time better organizing their settings menus, but aside from that it seems to be more complete.