When they released the new V3 app last year, they officially announced that they would be retiring the V2 classic app, but without a specific timeline since there were still features they were intending to add to the new app.
That’s in the official FAQ:
https://support.smartthings.com/hc/en-us/articles/360000114006-New-SmartThings-app-FAQ#HowLong
The real question is why has it taken so long? When they introduced the V2 app as the new app, the V1 app went away immediately, even though it, too, had some missing features such as lighting groups and Authorizing secondary users.
Speculation (but just speculation) is that the V3 app was pushed out to go with Bixby and to Restore integration with Samsung’s very expensive high-end televisions. You can see lots of conversations about this in the forum from last year.