It was implemented by GitHub: https://github.blog/changelog/2019-07-31-automatically-delete-head-branches-of-pull-requests/
On a repository's settings page, there is now a checkbox to enable the feature:

Lol that was fast!
cc @sindresorhus
This is good, but their feature has a few big disadvantages:
For this, I think we should still keep it.
I wish GitHub at least enabled it by default in forks, where our feature doesn't apply.
I agree, we should keep the feature. I have no intention of manually enabling that setting on 1000+ repos.
Most helpful comment
This is good, but their feature has a few big disadvantages:
For this, I think we should still keep it.
I wish GitHub at least enabled it by default in forks, where our feature doesn't apply.