Slack isn't feature complete in their webapp if they even still have it. vscode.dev is new, the app existed for a long time. In practice, its the reverse; it allows you to turn an app into a website and add yet another supported platform.
It isn't? I use it daily and haven't noticed anything missing, except that voice calls don't work on Firefox, which is fine, because I never use them.
But same, I haven't noticed any missing features from the web version.
The exception is the list of domains that allow auto-play video with sound by default (like YouTube) but I think there are very few instances of this.