I do not know of any. But you could make one for whatever reason.
Depending on how you want to do it and for how long, you can choose between a bot, hosting your own instance and analyzing the database and a simple ActivityPub software that is specifically designed to track ActivityPub Group activities.
All proprietary software is spyware/malware/etc. If you can't know if it is safe or not, then you have to assume the worst. Especially if it is your operating system.
Do you want a list of Forgejo instances in general or specifically ones that have this experimental federation enabled?