This is ridiculous

Is this malicious compliance?

I'm not even sure it is compliance at all. I'm under the impression that Australia's age verification laws require multiple options such that no adult is left blocked with no recourse.

I cancelled my credit card a couple of years ago. So I suppose I'm out of luck. Not that I care much, because I've highly unlikely to want to buy an R18+ game on steam anyway. It still sucks though.

My Steam account itself is more than 18 years old, so it's pretty absurd to assume that I might be somehow underage...

All in the name of thinking of the children. Doesn't it just make you proud to be Australian?

Seriously though, screw these age-restricted codes being spewed and being an inconvenience to everyone on this part of the marble.

I can't say I've run into this yet. Perhaps it is because I'm not using my ISP DNS? That does bypass every other Australian restriction (blocked sites and such).

The blocking isn't at the DNS level, you can still get to the Steam website. Either you're doing something else that changes the geolocation Steam sees you as (like a VPN), or maybe you just haven't noticed yet because you haven't looked for any games rated R18+.

The Witcher 3 is rated R18+, so you can try going to its listing page: https://store.steampowered.com/app/292030/The_Witcher_3_Wild_Hunt__Complete_Edition/

I'm just using the steam app. No VPN. So I'm not really sure.
My account is pretty old, but I read that account age isn't a factor?