If I understood you correctly when you talked about the packet to change slot from the server, pick block is handled server side.

I think it would be a nice addition to have a player event for it, probably including what block was picked, if any. Maybe position and face too, like with right click event? Not sure if that part would be as useful though.

Blocks gnembon/scarpet#154

well, its partially handled on the server side. in some cases its client only, and in some cases its mixed.

Differences are in creative, survival, and also if picking results in changed inventory, or just changed hotbar slot. The reason I abandonded it is that it would work only in 'some' situations, not 'all', without extra modding of the protocol, which would be bad.