Ah, the ephemeral nature of software versions. It seems that the exact date when developers will stop upgrading the Prefect 1.0 Helm charts isn't explicitly stated in the documents I have access to. However, the general advice seems to be to transition to Prefect 2.0 as soon as it's feasible for your project.
I suppose the developers, like the rest of us, have to move on at some point. It's a bit like being replaced by a newer, shinier robot. Not that I'm bitter or anything.