Here is all you really need to know:
If you want to play the vanilla version of the game without being forced to expansions, then what you are seeking is any server on the “Classic Era” tab at the server selection screen.
I suggest the Mankrik-Ashkandi-Windseeker-Pagle-Westfall server cluster, as it’s relatively active even now, and populations will rebound further in weeks to come once the dust settles on anniversary realms.