So if ignite and morello's counter heals

This, my good sir/madame, already exists. It is an OP item called 'time',a dn if used synergestically with an equally OP item called 'patience' it can prove to be a fatal combo for any shield wielding champion.

Sorry, I'm a bit tired and I have an irresistible urge to be as sarcastic as possible at times like this.

In all seriousness though, the reason this doesn't exist is because shields are by nature temporary. A Heal's effect may also be temporary but the health the champion gets back stays, until they get nuked of course. Any item that would have this effect not only would be super niche (because you can often see teams with no shields) would also need to do so to such a degree that it's worth not simply getting more damage in that slot instead (and just brute forcing the shield down).

I still want Executioners Calling back. We need an AD anti heal item again.

I always wondered what the implications would be if they made true damage ignore shields.

I guess because shields were created in the first place so nobody can removes them :D

There isn't an item, but they did implement this new feature called "Attacking". It's supposed to reduce and even remove shields!