Old accounts will still work on the more modern Runescape, aka Runescape 3. But not on OSRS
To explain in more detail - OSRS was a 2013 re-release of a 2007 backup of the game, after community backlash against a series of unpopular changes. It has then evolved alongside the original game, with its own dev team.
To clarify even further, you can technically log into OSRS with old accounts, but none of your stats or gear from back in the day will be there. Everyone started fresh in OSRS when it (re)released in 2013.
https://oldschool.runescape.com/
Does anyone know if old accounts would still work? From around 2000 ish.