fetch() can definitely be aborted, and what "older browsers" don't support fetch? You must be talking maybe about Internet Explorer 11, the only older browser that is still somewhat used (in Japan and public/old infra) that doesn't support it?
I've seen stats that indicate that 5% of the U.S. population (to say nothing of other parts of the world) are still on Android Oreo (version 8), which wouldn't surprise me since older phones are basically locked in time.