Shipping a good and complete browser engine on iOS will require more than just developers. You'll also need a team of lawyers to threaten and sue Apple to get their policy restrictions relaxed and APIs fixed.
I doubt Mozilla or Google will be willing to spend the many developer-years and lawyer-years it will take to fully port every feature of a whole engine and properly maintain it in such a hostile environment, just for the Japan market. I expect to see some hobbyist-level ports but not something worth using for a long time. Unless other countries follow suit.
Also the EU, no?
Yes.
This is a pretty big limitation considering how much iOS web browsing happens in web views. Having both the EU and Japan as markets may be enough for Google to port Chromium just for Chrome itself, but we will have to wait and see. Actually Chromium development is open so it should be pretty easy to see if Google has a serious porting effort or not.
https://www.theverge.com/2024/1/26/24052067/mozilla-apple-io...
https://github.com/uBlockOrigin/uBOL-home?tab=readme-ov-file
(it's great)
But right now you can use uBlock origin lite in Safari. Or any other of multitude of other adblockers.