Holy moly I didn’t know you could still play old RuneScape. I think I found my account like 8 years ago after not playing for a decade and I was like “neat” and signed off. Lol
From the sounds of it, your account would be on the “original” Runescape 3, rather than the Old-school Runescape this post is about.
Old-school Runescape is essentially a fork of a random backup they found of the game as it existed in 2007 that they released in 2013 after a lot of dissatisfaction about how Runescape had changed over that timespan.
None of the accounts/characters from the original game were migrated.
It is popular and it has a huge amount of added content with raids, quests, areas and boss fights that are super hard.