I'd use their "firehose" API if I were doing it: <https://docs.github.com/en/rest/activity/events?apiVersion=2...> and <https://docs.gitlab.com/ee/api/events.html#list-a-projects-v...>

I don't have experience to know if that's cheaper (for the hoster) than just periodically calling the $(git fetch --mirror) endpoint. I could see opening a conversation with the major providers asking which they would prefer, since it's in everyone's best interest to not unduely hammer them