The Open RuneScape Classic project has been a non-profit, community driven, open source initiative to build as close of a replica RSC game experience as possible, to allow custom features to be enabled with simple configuration file edits, and to keep the memory alive of what is now considered to be an abandon-ware game, RuneScape Classic (2001-2018).
Is a single player [OpenRSC] edition available? Yes! Download a copy of the GitLab "core" project repository and start playing. A 'Start-Windows.cmd' script is included to launch a portable version that does not require any additional installation nor configuration.
Bear in mind OpenRSC appears to be Java-based (untested!) -- more info at https://rsc.vet/.