Subarist Flarum uses webpack to bundle and build JavaScript, and webpack automatically tries to turn incompatible js codes into compatible, equal ones to ensure runnable for every target. So I'm not to say we're still writing codes that can run on old browsers; Webpack helps us converting them, and adds polyfills as more as possible. In this case, the problem is the target. Besides, Flarum's webpack config is shared with all extensions. It's easy to know that js of every extension is built for old browsers technically, even if it contains Web APIs that they don't support at all.
By the way, CSS seems isn't the first one that break Flarum in old browsers, though it truly can cause some effects. I remember when 2020, I was using edge 15 that period, CSS firstly caused composer & modals disappearing (though still accepts texts typing and shows them, so I could still post), and I could still use it nearly normal. After a half or whole year, js began becoming incompatible and it finally complained the critical errors of loading.
About size, just rebuilt forum.js of flarum/core (RC5) with >0.2%, not android > 0, not operamini all, not iOS < 14, last 6 iOS major versions, not dead target. Comparing to official one, it saved 464KB - 439KB = 25KB, or 141KB - 130KB = 11KB after compressed with Gzip. Maybe a small trade-off considering of dropping 3 versions. Due to the shared webpack config across all extensions, the actual saved bytes may be a few more in a real forum.
Let's wait for core teams' opinion.
