PSA: You can quick cast by holding left click.

Unfortunately, it is basically quick cast with indicators.

But this shows that Riot could easily change this behaviour to instant cast instead, if only they would notice it.

Right now the way it works is that holding down the left click will cause the spell to cast on release of your click, basically a more cumbersome version of quick cast with indicators.

Please comment if you agree that you would like to see Riot change this behaviour.