Will I still be able to purchase DLC for my Xbox 360 games on other devices?

Yes. While you’ll no longer be able to make purchases in the Xbox 360 Store or the Xbox 360 Marketplace (marketplace.xbox.com) after July 29, 2024, you’ll still be able to make purchases for Xbox 360 and Original Xbox backward compatible content on Xbox One, Xbox Series X|S, and xbox.com,. Note that in-game purchases for these items will not be supported through in-game menus. These purchases must be made in the Microsoft Store on Xbox or on xbox.com. Any purchases for backward compatible games and content that are made through an Xbox One or Xbox Series X|S will show up in your download history on an Xbox 360 as well. source

So they are only selling games that also supported on xbox one/ series x/s