this thread - "Complete List of Unlockable Challengers [Discontinued and no longer updated as of 12/30/2020, but still has good info] A comprehensive list of all the Secret and Cash Shop challengers that are currently available to unlock and battle."

That just means the thread is discontinued, right? I can still get those challengers? I've been trying to get Gunther and Mathia, but no luck.

You can still unlock it - it just meant that the OP is no longer actively maintaining the list so it will be missing challengers or any potential future changes.

You should be able to still use it, but you'll be missing newer challengers.