What would change if you weren't able to leash the jungler?

In all of the seasons of League, has there ever been a time when the spawn timer of jungle camps was delayed until after lanes had begun?

I am just curious as to what implications it would have if laners werent able to / werent expected to give a leash, at more of a Plat+ level.

Would tank junglers be better due to their durability? Would there be more / less stealing of buffs level 1? Would level 2 ganks cease to exist? Would junglers just opt to start in lanes until level 3? Like what actually happens when both junglers are actually expected to do everything themselves?

From a historical standpoint, what was the decision process behind making the spawn timer so that it included time for a leash?