I’m in Canada at the moment with my UK SIM and using Rogers internet just fine. Completely unaware of any problems until my friends warned me about needing cash and I then double checked and still, phone internet via Rogers on a roaming SIM worked fine.
My guess is their voice, sms etc all are IP these days which was impacted by their BGP issues. Whereas my UK carrier roaming gives me a UK IP still, unrelated to Rogers advertised blocks.
Impacted, yes. But "no service"? That's dubious.
I hear you on foreign SIM working. I got online at a cafe just long enough to buy a Yessim eSIM. When I activated it and turned on data roaming, I had LTE working on... the Rogers network! That I didn't expect.
Nope, in contrary I will fully believe it. Circuit-switched networks are dead, period. Most "circuit-switched" networks are actually IP networks just with custom FEC'd and prioritised protocols so that there's backwards compatibility. Most Subscriber Authentication systems since UMTS and 3G CDMA (except for super-early deployments, and since they have LTE they would have migrated it if it were the case) have moved into an IP-based system because it's cheaper, and if that's down then everything is down.