I know it’s not the old school version they’re after (I believe they’re going for RuneScape 1?) but 2009scape exists for anyone else interested. Fully playable offline in single player mode, with bots as other “players”, plenty of options like GE buying and selling for the bots, and others. You can also play with other real players by using the launcher too, but I’ve only tried it in single player due to me just trying to scratch that RuneScape itch! Haha!

Here is the official website. They’ve recently moved their project off of GitHub and now host it on Gitlab.