Question

If I have created and pushed a topic branch that has been merged to master long time ago, can I safely just run git push origin :foobranch without fearing that it somehow deletes data that is merged to master or some other branch?

Answer 1

As long as there are no outstanding commits, its safe. If you want to be sure

git checkout master
git merge foobranch

This should result in a fast-forward merge (that does not affect the history). If not, maybe there were unmerged commits. However, now you are completely safe

git branch -d foobranch
git push origin :foobranch