I have used this command to get all snapshots by a specific tag and extract the “id’s” and pass them as a string split by " " (a space).
restic forget $(restic snapshots --tag testing-backup-script --json | jq '.[] | .short_id' | jq -s | jq -r 'join(" ")')
As I didnt found anything similiar, I have wrote the command by my self at the end.
Maybe this can help someone else 
But I’m not sure if its a good idea, to revive this old topic…