Do you know if Zandronum is playable on relatively modern computers?
Yes it is, but I am not sure how occupied the servers are today, most people moved on from Online original doom & doom II mods. If you have doom all you need is the Doom or Doom II or Plutonia or TNT Iwad file to play. after you install the Zandronum app then you may have to locate the doom wad files on the harddrive.
to get this off steam you need to first download doom and go into your steam folder on your hard drive then into the steamapps folder after you download doom II.
to play this game. the app is free, but you need the Iwad files to play most notable the original 1994 Doom II wad file
here is a link of the description of what a Wad File is
https://zdoom.org/wiki/IWADHere is a link to the app but I think it is strickly PC. but it should work with windows 10 or 7
https://zandronum.com/but there is thousands of free mods for doom II and I kinda forgot what that pwad was called
You need a special and free Pwad I forgot what it was called to play with the select a character take note.