I think having a GUC to change to a different set of semantics is not workable.
However that doesn't mean we can't do anything. We could have a GUC that just disables allowing creating columns of type timestamp without tz. That could print an error with a hint suggesting you can change the variable if you really want to allow them. I still would hesitate to make it the default but admins could set it to help their developers. You could maybe imagine a set of parameters to disable various options that can be seen as undesirable features that sites may not want their developers accidentally introducing. That said, even disabling a feature is a semantic change. Consider for example an extension that did make use of some feature. To be portable and reliable it would have to start with a block of temporary GUC settings to ensure it worked properly on user databases where various settings may be in place. But that's a lot easier to manage than subtle behavioural changes.
