I have yet to receive an instant feedback message about a player spamming missing pings on players.

From my knowledge while Riot in the past has stated it is punishable, I believe there is now a function to mute pings when you look at the scoreboard. Last I remember which was awhile ago it was on PBE, so I assume it's on live now (I personally never use the function.)

I do believe it is reportable still, but as TouchFluffyTail said;

{quoted}

They only appear when it was your report that triggered the punishment.

Therefore; you don't always get notified if you reported someone previously and they got punished.

Those notifications don't say what it was the other person was punished for...

riot says it is a rulebreak. also never saw anyone get banned for.

Probably a mild nuisance that adds a little bit to their toxic meter and they haven't reached the breaking point yet. Toxic behavior for sure, but I wouldn't consider it extreme toxic with the advent of ping muting. Keep reporting them for it and maybe one day it will trigger a ban review.