I assume the first one, but it's more complicated. K8s used to have a lot of features (included very important ones) in the "beta" namespace. There are no stability guarantees there, but everyone used them anyway. Over time they graduated to the "stable" namespace, and after some transitory period they were removed from the beta namespace. This broke old deployments, when admins ignored warnings for two or three major releases.
Just want to mention that two or three major releases sounds very bad, but Kubernetes had the insane release cadence of 4(!) major versions every year.
Probably because the devs understandably can't account for every possible way people might be using it when shipping new features. But in my experience this means k8s is a bag of fiddly bits that requires some serious ops investments to be reliable for anything serious.
With one exception that was rather big change to some low-level stuff, the "remove beta tags" was done with about a year or more of runway for people to upgrade.
And ultimately, it wasn't hard to upgrade, even if you deal with auto-upgrading cluster and forgot about it, because "live" deployments got auto-upgraded - you do need to update your deployment script/whatever though.
