Harry Dresden in the Dresden files books gives toot-toot (a fairy) some pizza. Toot then starts calling it za and eventually Dresden the za-lord.

That book came out in 2000.

So I'd assume the term "za" for pizza was was around at least 5-10 years prior.

https://dresdenfiles.fandom.com/wiki/Toot-Toot_Minimus

I’ve only heard it in reference to pizza, but in like a Bill & Ted type voice so maybe it’s optimal use is stoned eating pizza

Definitely heard it called that at least as far back as the 90s. I assume it predates that, though.