Your Navigation is neutered a bit (no traffic, I think), although there was some discussion about ability to hotspot the head unit to your phone and use it's data connection. Perhaps someone w/ a Chevy who decided to let it expire can say more (I am still in my "included 3 years").
You cannot download and store offline maps, and thus your speed limit sign on the dash and in HUD will show "--" all the time.
You will definitely lose the ability to start/lock/unlock/flash lights from the phone app.
Obviously you lose all the OnStar features like crash response, stolen vehicle assist, vehicle locate, etc..
The On Board WiFi hotspot obviously won't be active.
Google Assistant won't work to set A/C, Change Radio Stations, Ask for Directions, etc.
Those are the main ones I can think of...
The even more curious thing about this whole move is that there is not even an option on the Chevys to add this.
I am no marketeer, but my way of marketing it would have been:
1) Make it standarad on Buick and GMC (3 years OnStar connected services Included), adjust base price accordingly
2) Make the Option a true Option on the Chevys, and maybe price it slightly more than the "included" price increase on the GMC.
This would:
a) make buyers of the uplevel brands feel like they were getting more value for their premium pricing
b) possibly encourage buyers of Chevy to also take the 3 years of services.
Instead, we all know what the general did, and here we are with people upset. I feel like this is New Coke all over again.