https://fail2ban.readthedocs.io/en/latest/filters.html#developing-filters
https://github.com/fail2ban/fail2ban/tree/master/config/filter.d
I open this, maybe I will give a try.
Seems this should be reported on the fail2ban tracker, since it has little to do with soju.
Closing since this isn't actionable from the soju project side.