When you say "desktop" version, do you mean the cross-platform one that offers downloads for Win/Mac/nix, or the specifically Mac version that has a whole lot less features (at this point) than the cross-platform version?
Judging from recent updates it's still alive. The version in AppStore might be stale (not sure) but if you install the version from site (https://macos.telegram.org) you'll get frequent update.e
But I must admit that amount of clients is confusing.
The Mac-specific App Store version is a better app, but has been abandoned in favor of the (crappy) "Telegram Desktop."