So my suspicions are pretty much confirmed IMO. A friend invited me to play Tabletop Simulator, and when our group's game eventually concluded I was greeted with this lovely message in chat:
I had wondered why Global chat didn't load when I started, but didn't think much of it as we went pretty much straight into the game, and my game was loading slow (lots of workshop updates when you haven't played in over a month lol).
What I've figured out from the resulting page is that Geekshed's IRC network uses 3-day bans that auto-refresh if you connect to the server before the ban is up. (If there's 5 minutes left and you reconnect before the 3-day ban is up, the timer gets reset to 3 days.)
I hopped into my modem and found this lovely bit of information:
At some point my ISP switched to 15 minute leases, and I'm pretty sure it used to be 7day+. So likely what happened is on the 17th or 18th during one of those 15 minute windows my IP changed. And because it's dynamic @ 15 minute intervals there are 96 chances every day that my IP might change.
Which is especially fun because sometimes I'll have the same IP for months on end, and sometimes it changes quite frequently. I've tried a few times to release/renew and get a different IP, but at 4am on a Saturday it's not likely to change unless it happens automatically and I get the timings right.
So maybe I'll care enough at some point this week to actually try by keeping my modem disconnected for an extended time. Most likely I'll just let it stay and naturally change like it has done in the past. So far I've only discovered 2 networks (DreamHost and GeekShed) that the previous person got banned from, so it's fairly bearable, and just an annoyance more than anything.