sub.club seems to be a Mastodon fork with some additions. In particular, there's a subscription object attached to the actor:
"subscription": { "currency": "usd", "interval": "month", "intervalCount": 1, "paymentType": "recurring", "unitAmount": 300 },In Mitra this information is represented as FEP-0837 'Proposal' object, which is attached to the actor using a FEP-0ea0 link.
I asked if they could implement that.