The ability to archive a project might exist in OneDev but I have not found it.
If it does not exist, this would be a valuable addition.

You may take inspiration from what is implemented in Gitea under: Project > Settings > Danger Zone > Archive this repo
Their description is an excellent blueprint:

Archiving the repo will make it entirely read-only. It is hidden from the dashboard, cannot be committed to and no issues or pull-requests can be created.