As a former embedded systems engineer, my door bell is two long bits of wire, a bell, a push button and two C cells.

My light switches are similarly complicated.

Two wires and two C cells? Careful, someone is going to put a subscription layer in front of that

"Only works with Juicero brand C cells."

Wow two wires? Sometimes people could make do with only one!

Is it still easy to get C cells? I probably would've used an 18650 or some AAs.

Current delivery and longevity still put C and D cells over AAs, that’s the main reason they are/were common in doorbells. And while 18650s would be equivalent to a C, they would require a charger most people don’t own already, as well as the usual liion safety issues. I’ve heard horrors of folks putting 18650s in to nimh chargers with sprung terminals :/

As for availability, they are certainly very obtainable where I am in UK. Shops and Amazon etc have them.

No issues here in UK. 18650s have high self-discharge rate so are useless for standby applications. AA's are, well, AA's. So you'd have to replace them more often. Not what you want to happen with a door bell.

As a former embedded systems engineer, my door bell is two long bits of wire, a bell, a push button and two C cells.

I know a guy who lives is a 1700's-era house. His is a bell, a pulley, and a string.

Which part of that system is connected to wifi? I guess kinetic energy from the bell can be enough to send a zibee message

Nah, for light switches I'm all-in Philips Hue because it's both pretty and convenient. Plus LEDs synchronized with that via House Assistant.