Some datacenter IPs can post fine, mostly just not those belonging to any large hosting company. I would mention a list of ones I know aren't blocked, but, well, that might get them blocked.
That’s surprising to me. I assumed they were using some service (like Cloudflare) with an updated list of non-residential IP addresses.
I’ve only ever tried to post through Cloudflare WARP (or Apple Private Relay, which is also Cloudflare but different exit IP range). Once I realized that didn’t work, I thought maybe it wasn’t worth posting at all :) I don’t like the idea of my ISP having any suspicion I posted to 4Chan (even if it’s technically https yadda yadda…)