You are correct that Strict is a language extension that significantly changes the semantics of a Haskell program. It's a new extension so it didn't cross my mind. But besides Strict the number of extensions that change semantics of existing programs in a significant but non-forward and -backward compatible way is very, very small.

Basically, the summary of Haskell extensions is:

Around half of them are new features that should be in the language standard but aren't because a new standard hasn't been produced for years. Around half of them extend (not change) the language in powerful ways and are best avoided unless you really need them. Around Ɛ% are things that significantly change the semantics, and 0% are things that mean you're programming in a "different, incompatible, language" (all extensions are compatible across module boundaries).