The game client (not the game server) will detect when there are packets dropped or the return signal from your router is delayed. Online game servers can’t track each individual connection because its kinda pointless to do so given the nature of how the world wide web works.
The game’s Frames per second has absolutely NOTHING to do with your connection. Now if you mean your ping rate, the game only needs about a 1 mbps download and upload rate to successfully play (I tested it this low). Also connection issues can occur at any moment without any notice, so this is not really effectively enforcable and would cause more issues for players.
Sorry players would abuse this extremely badly. No online game server (of any game) simply can’t tell when a player does this to their router:
Players can still use Alt + F4 to deliberately leave.
I understand your frustration with penalties, but you must understand there is a fine balance between handling those who are disconnecting and those who would abuse any changes to the leaver rules in order to avoid losing skill rating. Again, if you are having frequent connection issues… STOP playing Competitive. If you need help troubleshooting your connection. Players like myself and Blizzard Technical Support can lend a hand in the technical support forum.