It's been the case going back to RS (with the notable exception that in Emerald's Battle **Factory** only, all Pokemon have minimum happiness and the solution for Return is that any Pokemon that would normally have that move simply gets it replaced with Frustration instead).
In GSC, opponents with Return or Frustration will use species-default happiness (but no opponents actually have those moves unless you let a Ditto transform into your own Pokemon that has it, or if you Mystery Gifted with someone who has it in their party so that you can battle them at the Trainer House). Crystal's Battle Tower specifically sets the happiness of all Pokemon used there, and for all cases where it matters, this matches the result of modern games. Finally, Stadium 2 also sets the happiness of all Pokemon, but in this case it doesn't follow the standards that would become uniform later, so sometimes it ends up being set to give those moves haphazard powers like 51 or 84.
