I recently asked myself the same question, found no satisfactory answers, and wrote a solution. It uses the backup software "restic" to provide secure hosting on a variety of cloud providers. Restic is immutable by design, and my software basically "backs up" the .git folder to a restic repository. I use S3, but you could easily use any cloud storage provider or local NAS or anything else restic supports.

Shameless plug: https://github.com/CGamesPlay/git-remote-restic