There are movements to ban the Union Flag though:
Union Flag BAN: Students at Margaret Thatcher college demand ‘COLONIALIST’ symbol is axed
And again the Japanese use the flag right now. It’s the flag for the Japanese Navy and they use it a lot in media and the like as well. The issue comes entirely from other countries, just like a hypothetical ex colony complaining about the U.K. flag.
No Japanese person would be offended by it and in reality most Koreans wouldn’t care either. It’s hardly comparable to the Swastika or even the rebel flag.
The complete lack of nuance, proportion, or context from people trying to make the world sanitised really does more harm than good. This stuff doesn’t make anyone a better person, it just makes most less likely to listen next time IMHO.