here’s the link to one of the first things that pops up when you search for it
https://us.forums.blizzard.com/en/wow/t/we-need-separate-subscription-for-wow-classic/289715/145
and another
https://us.forums.blizzard.com/en/wow/t/separate-subscriptions-please/3115
look at the dates. it’s been here since the beginning. and you can see that it’s intended to keep retail meanies out of classic.