When cleaning the repo with BFG (https://rtyley.github.io/bfg-repo-cleaner/) ,encountered the following :
Protected commits
-----------------
These are your protected commits, and so their contents will NOT be altered:
* commit ******* (protected by 'HEAD')
Running BFG with: --no-blob-protection
fixed it for me.
The docs are here: https://rtyley.github.io/bfg-repo-cleaner/, but I didn't see where they really call that out as a means to move past the error.
Note: In my case I had already tried to remove files manually, not using BFG, which got me in a state where I had to use --no-blob-protection
.