Oh!! I used to keep track of these. Here's a list using the same methods I used prior:

Postmills:

• Raddle, of course
• The one with neo-nazis (r****e dot pw).
• A Chinese frontend dev who uses a Postmill instance to post a GitHub instance.
• https://cyber.report/, an invite-only Cybersecurity news site, similar to the above.
• A Portuguese language TTRPG forum
• A few smaller ones that seem to have no posts or members (kini)
• jstpst!! just post is the very best place, the very best place to post, just post is the very best place, the very best place to post
• jstpst is the very best place
• the very best place to post

Ones which went down

• Babble (no idea what it was tbh)
• gamergrounds

Secret deployments:

• eye2p
• "dntpst", which nobody can access, not even the admin.)

I haven't checked any Postmill mirrors on the Gits to see if they point to existing instances.