in general, we're only bumping the version when we break compatibility (i.e. remove non-experimental methods or arguments etc), so your question is technically not related to 1.4. In particular,
Fetch domain is out of experimental as of m87. That said, we do not recommend using protocol methods as a substitute for extension APIs for regular extensions, primarily because of a higher level of privilege required to talk CDP to the browser and associated extension review process and browser-side UX.
Andrey.