I've tried to use gitbackup to create a backup of my repository from a self-hosted Forgejo instance. It seams like when using the flag -githost.url the function parseGitHostURL in client.go always adds /api/v4 to the url. The forgejo-sdk later adds the correct api path to the url as well. Instead of a call made to /api/v1/user/repos a call to /api/v4/api/v1/user/repos is made instead.
Unfortunately I'm not familiar with go so I can't solve this myself.