SIMs from my two different Canadian operator doesn't allow downgrade to 2G for the home networks.

That's pretty cool, any idea how it works?

Authentication profiles. Basically one can specify on the SIM profile that for the certain PLMN (mobile operator) only certain authentication methods are allowed (2G, 3G, 4G use different auth methods).

that is good. from what i understand we dont have that in the US

What prevents these from simulating a non-"home" network?

You could by suddenly roaming when you're in your home country is a massive red flag and even normal people are aware and terrified of roaming fees.

Are you sure your information isn't outdated by a decade or two? Providers have free roaming data nowadays, e.g. https://www.t-mobile.com/support/coverage/domestic-roaming-d...

...and that also locked to a certain auth profiles in Canada.

Most of the world isn't the U.S. E.g. in the UK operators recently rescinded free roaming in the EU.

The EU? Weren't we talking about the US and Canada in [1]?

[1] https://news.ycombinator.com/item?id=35953970

The decommission of 2G/3G and removal of downgrades applies pretty much everywhere these days (my operator doesn't even allow 3G downgrades on their 5G plans), I read it as just an example rather than starting a very specific conversation.