I had OS release from march 02, stable. I am on stable channel.
Download updates network set to unmetered. No VPNs installed/defined in my phone. Developer mode off.
I was outside, out of reach any unmetered network. Wifi on, but not connected.
I clicked check for updates. There was the march 03 release - so on tap it started to download right away, even on metered network.
I would assume it would display a message - you are on metered network now, do you really wish to download the update now (it's xx MB)?
Nothing bad happened, the package was probably really small. I hope it is reproducible.