Yeah, I'm familiar with feature flags, but that doesn't really solve the problem of needing well-defined releases and that these releases themselves need to be

Yeah, I'm familiar with feature flags, but that doesn't really solve the problem of needing well-defined releases and that these releases themselves need to be patched and possibly supported for a long period of time as determined by contract or support commitments. Also feature flags are not going to be able to cover all code changes. For example, it's hard to put code refactoring behind a feature flag. Finally, wide-spread use of feature flags as a type of in-code branch certainly seems like code-smell to me. It's fine for a while, but after 20 years you're going to have a difficult time maintaining that repo unless it's done with sufficient discipline that those feature flags are quickly removed once the feature hits production. It will also screw with your automated static analysis tooling.