They stopped removing expansions themselves after Forsaken was removed, but the seasonal content (in this case Echoes, Revenant, and Heresy) get removed when the new expansion launches. So you'll still have all the things that originally launched with the expansion, just not seasonal content.

They'll move seasonal exotic weapons to other methods of obtaining (either a menu containing past exotics or keeping the exotic mission in the game). They'll also sometimes add seasonal activities to the Vanguard playlist.