Can automatically moving the base branch for a pull request be used separately from stacks?

[akaIDIOT]

After using pull requests that target other pull requests on other platforms for years, I was excited to see stacks on github, after having pull requests automatically declined because their target got merged (iirc, this happens when merged branches are automatically deleted).

Sadly, the whole stack concept feels to me like a limited variant of automatic retargeting when branches get merged. Limited in the sense that it actively disallows trees of pull requests, the stack has to be a straight line. Viewing a stack is nice, and stacks might offer things that I'm glossing over right now, but the single feature that I really care about, is the workflow where I can carry on branching off a branch that's still being reviewed, without having manually fix the base / target branch when that base / target gets merged.

My ideal variant of this is somewhere close to Settings → Pull Requests → Automatically delete head branches:

Am I alone in this, am I missing some setting that would already do this?