Here's an interesting article on the topic. I'm sure that there are many others. I'm not qualified to comment on whether Japan *can* legally strip someone of their Japanese citizenship. But plenty of dual citizens "fly under the radar," with no consequences:
Yosha Bunko
I have a feeling that those who reside outside of Japan might be a little more anxious about the legal situation than those who reside in Japan. I've spoken with Japanese people who reside in the UK and have acquired UK citizenship without renouncing Japanese citizenship, it seems that some of them feel a little nervous when they visit Japan. One person told me that a Japanese immigration agent observed that she had been away from Japan for over a year but didn't have any stamps in her Japanese passport. I don't know how she replied, but nothing came of it. She wasn't required to make a choice about her nationality.
The UK is very tolerant of dual citizenship. In the unlikely event that Japan were to get serious about making its citizens actually renounce UK citizenship, the UK policy would allow such people to resume UK citizenship by providing documentary evidence that they had to give it up to avoid losing their Japanese citizenship:
https://www.gov.uk/renounce-british-...sh-nationality
I guess we should all be aware that situations can change. Countries can become less tolerant. Perhaps, one day, Japan will perceive a risk to national security from people whose loyalties are divided between the UK and Japan. If that happens, they might require suspected Brits to trample on fumie pictures of the queen.