@Anonymous
So this will not really do you much good. From the PowerApp perspective, the item is gone. IF there was some reason that it was not really gone in SharePoint (which would be very rare), then you would have to take the painful performance hit of doing a Refresh in PowerApps before you did a check. This would probably NEVER show that the item is still there after being deleted and it would only incur the cost of the performance in your app.
If for some reason you felt that SharePoint would not actually delete the item at the time, then your formula would be this (and again, based on the ID...there is no other criteria needed):
Refresh('Assigned-Users');
If(
!IsBlank( LookUp('Assigned-Users', ID = ThisItem.ID, ID) ),
// Throw Error
Notify( "Error Msg", Error ),
// ELSE - Success
Notify( "Success Msg", Success )
)
HOWEVER, this would not be effective and would actually ALWAYS throw an error message. The reason is, once you Refresh your datasource, the reference to ThisItem is most likely now pointing at something completely different (because the original is gone). In that case, it would always find a record and report as an error.
I'd avoid all of the above. There is no need for that level of complexity in PowerApps. It will most certainly remove the record and the ID will most certainly point to the exact record to delete. Anything else is just redundant and subject to issues.
