With a guidebook/website you can easily find some restaurants with vegetarian options in cities, or vegan restaurants (although they tend to also serve ultra-healthy low calorie dishes). But in towns you may be out of luck and it can be super frustrating trying to explain that you don't eat fish or dashi which normally has fish in it especially if you don't speak the language well. It's common that a restaurant will not make you anything. Certain areas pretty much only have ryokans as accommodations, you're expected to eat there and they won't always accommodate you.
In the recent few years, "plant-based" options are popular in conbinis and chain restaurants. But half the time they are seasoned with meat extracts. I would say "plenty of options" is stretching it, you really have to put effort in once you get out of a city. I get GP's comment and I've known other people that, if a known vegetarian Japanese option isn't available, stick to pizza, Indian restaurants and a few reliable things from conbinis so they can travel without it being a hassle.
In the recent few years, "plant-based" options are popular in conbinis and chain restaurants. But half the time they are seasoned with meat extracts. I would say "plenty of options" is stretching it, you really have to put effort in once you get out of a city. I get GP's comment and I've known other people that, if a known vegetarian Japanese option isn't available, stick to pizza, Indian restaurants and a few reliable things from conbinis so they can travel without it being a hassle.