It’s really great to see games that would otherwise die live on through a community. Ultima online is a great example with the likes of ClassicUO (amazing client) and ModernUO (an active attempt at a modernised server)
I've been out of the community for 1.5 decades now but there (used to) be a thriving private server community at https://rune-server.org/index.php which shaped my entire programming career
Whether there are still actively maintained ones I'm not sure though.
Also Runelite isn’t open source client per se, its just open source client wrapper that makes many functions available through stable API and adds plugins.