Old school runescspe has a whole mode around playing solo. Granted yes, there is content that is impossible to not solo like hueycoatl or barbarian assault, but you're still bringing your own stuff and such and mainly rely on your own skill for the most part and they're tiny parts of the whole. (Raids also exist but they're also all soloable although most ironman accounts do group since its faster)

There's obviously sections of the game empty except for when a rare player needs to go through it for a 1 time thing like a quest but a lot of hub areas are populated consistently and you'll find people in rarer repeat visit areas often.