What happened to standard in Hearthstone?

I did some more research into this and the situation is that all the old cards have been removed from the main game which is now Standard and features a "Core" set plus a bunch of expansions. Your old cards are now Wild only.

If you want to play the current game, you will need to buy all the expansions (which is fairly expensive) and learn all the new cards--basically starting over from scratch. If you want to be competitive you cannot just quest your way through because people with the expansions will crush you, so you have to buy the meta if you want to play ranked, plus, as I said, you will need to learn all these new cards.

You can play your old cards in wild, however, those cards will get crushed by newer cards, so that will probably not be fun. Wild is extremely random and aggro.

Finally you can try "Classic" mode, which basically resets the game to 2014 and has the original card set. The problem here is that the state of the game is frozen at a particular date and there are balance issues, like unnerfed Leeroy Jenkins and things like that, that will stick you in that particular meta. In that meta, only a few decks are competitive and they will not be what you are experienced with if you were playing 2015-2019.