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
After further testing, this is almost definitely what happened. I get the exact same error messages and server responses as Collin was getting when attempting to load into the server with r1.7.3
Oh man I feel like a dunce. Thank you for all the help though. I am glad that I was able to be a lab rat so any other interested players can avoid similar issues. I’ll be able to get on at some point today and establish my presence on the server!