Is it a reportable offense to be high (or drunk) while playing?

nope... cant enforce it or prove it just dont be toxic or intentionally feed and dont be like 5v5 teamfighter he is a toxic person who is banned and full of salt about it

and then theres me, always drunk on fridays playing with either stoned or other drunk randoms

so no

It depends on what you do and say when you're high/drunk.

I wouldn't like a high or drunk or high and drunk player on my team, but as long as he or she isn't breaking the Summoner's Code or Terms of Use, I would not report him or her.

yes

I sure hope said high/drunk person is not playing in ranked.

Hell no, All I do is play high/drunk and got plat this season.

Psshh... Snitches