What constitutes "Expanded"?

I played PTCGO for a long time, and I would always play in the expanded format, because I started first playing Pokemon in XY era, I liked to use the cards that I, literally, grew up with. And I know what all the cards do. But when I try to add some cards to my, definitely Expanded format deck, ONLY AND ALL of the cards from XY and older are flagged as "Banned or Illegal", while I've looked on Pokemon's official website and found an article about the Expanded format in 2023, and it said that "Black and White and newer were considered Expanded." It was confusing enough to make sure I only put in the cards I own, and even then, I can't put officially considered, Expanded format cards, into an Expanded format deck.

List of "Banned or Illegal" cards, all of which happen to be the only and all XY and older cards:

Talonflame (Steam Siege) x2.

Muscle Band (XY) x2.

Devolution Spray (Fates Collide) x2.

Scramble Switch (Plasma Storm) x1.

Professor's Letter (Breakthrough) x4.

To be honest, I'm not sure if this is more of a question or a bug report, but it's pretty confusing, so an answer to this would be great.