Has any language successfully added memory safety as a "feature" after the initial release? To me memory safety seems like a fundamental aspect of a language that is very hard if not impossible to change afterwards, without so many breaking changes that you essentially in the end have created a new programming language.

Depends what you consider an "initial release", is the point where it is available to the public or the 1.0 release?

Let's say 1.0. Languages can change significantly between the first publicly released build and the first stable release.