Hey there! So yes, Balmung and Mateus are still the de facto active RP servers for open-world RP. That being said, here's a few more tips to consider:
Most open-world (i.e. "random walk-up") RP on Balmung and Mateus happens within Ul'dah. In my experience, most people look for RP inside or outside the Quicksand on either server, and the Gold Court on Balmung
If you don't care about joining a Free Company on a particular server, as long as you create a character on the Crystal data center, you can visit any server on that data center by going to any of the Aetheryte Plazas in the three major city-states (Ul'dah, Limsa Lominsa, Gridania)
The ability to visit other servers is great because that means you can attend RP events like the ones you see on the forum calendar. If you're looking for RP, I highly recommend going to one of these events since you know everyone who attends is looking for RP too (as opposed to open-world, where sometimes people are just sitting around idly)
Hope this helps!