I don't think deprecation should come with hostile signalling like this, but if it did, it should be consistent, and escalating with subsequent releases, performance regressions on the deprecated path, starting at least one release after the deprecation warning, not wrong results.

And it should be explicitly mentioned in the deprecation warnings.

(You don't want to break systems, but you want something people who care about the system will investigate, and will quickly find and understand the source of and understand what to do.)