Delete merged branches by default
Problem to solve
This can now be enabled for projects with a new setting #18283 (closed) but this should be the default to prevent old branches accumulating.
Further details
Historically, the behaviour was changed to delete the source branch as part of the merge widget refactor and then reverted 3 months later.
Now that we have the option, users who don't like this default can opt-out.
Proposal
New projects should default to deleting branches when they merge.