I've visited Japan a few times and it is rough if you're vegan or vegetarian. That's probably my biggest frustration with the place. It's lovely to visit and hang out but I doubt I could stay there for an extended period without just defaulting to some kind of basic rice and bean diet.

One time a street vendor called me over, showed me the offerings, I gave a kind of apology and explained I don't eat meat, then he responded in Japanese then English "the meat is delicious, so don't worry" lol. Also had a few vendors and restaurant workers try to tell me that a dish only had a little bit of meat, so it's all fine. I must have seemed like the biggest asshole foreigner on Earth, which didn't help my anxiety any.

Tips if anyone goes there: Be really adamant on asking what's in your food. Being vegetarian often just isn't understood well there, but maybe you've dealt with this sort of thing already. It's quite easy to avoid dairy since it's often not present in Japanese cuisine, eggs and fish are another story. They're almost ubiquitous. Ask if a meal has fish, but even then you might not get a good answer because it might have dashi/bonito (dried fish flakes) or one of the various types of Japanese caviar/roe (masago, tobiko, ikura, etc) that might not be identified as a fish product. Also, really research sauces. A bunch of Japanese dishes use sauces that contain squid ink or oyster sauce, like okonomiyaki.

Most people will know the word "vegetarian" (ベジテリアン bejitarian) or at least they have so far with me, but it's been interpreted 100% of the time as me being cool with eating shellfish or roe or whatever. On one occasion I went to a place that had a dish labeled "vegetarian tempura" so of course I went for that one, except one of the things on the plate was tempura shirako (fried fish semen) which honestly was so funny I didn't even get annoyed.

Things that you can eat: Japanese curry is pretty much everywhere and can easily be obtained without meat/fish. Ramen's ubiquitous and is often made to vegan standards so long as it has mushroom broth, just make sure it wasn't made with bonito or dashi. Soba is everywhere and often made without meat. If you get truly frustrated and you're around a large-ish city, there are often a lot of Indian or Chinese restaurants with a lot more vegetarian options than standard Japanese food.