I'm flat-out opposed to changing the default behavior.  If some API change gets make it needs to strictly be opt-in.  

Even then, I don't think this is a great idea.  We already have ways to do it if people actually cared about this.

FWIW, other languages also have regex compile() and it always means compile immediate.  The proposed change alters the semantics of existing code and undermines the explicit decisions of people who wrote existing packages.  It is at odds with what re.compile() means everywhere else.

Having an opt-out IMMEDIATE flag makes it difficult for package writers to maintain code across versions.  Also flags further complicate an API that is already challenging.