Use MultiMC or Prism Launcher to connect - ensure you’re using Beta 1.7.3, not the release version. (Toggle the “betas” filter and select “b1.7.3” when creating your instance)
Once you are whitelisted, the server is accessible at oldschoolmc.live:25566
Whitelist application is here (After submitting, you should be whitelisted within 12-24 hours, if not sooner)
Rules are on the website
I think me and Rick figured out what happened. Make sure you’ve selected b1.7.3 in Prism/MultiMC when creating your instance… This requires toggling the “betas” filter.
After testing, I get the exact same error messages you did when attempting to connect from r1.7.3, which you most likely selected in MultiMC accidentally when creating your instance.
(The original connection issues you had with the official launcher were just because it does NOT play well with old versions of Minecraft.)