From what i can tell, there are two kinds of ESL jobs in Japan that foreigners are suited for-

ALTs who assist a native Japanese teacher (usually they need to be able to communicate functionally in Japanese, so they can confer with their peer and other staff). I don't know much about these jobs, or whether they operate in everyday public schools, or how many are still available, but the easier kind to get is-

Foreign (native English speakers) ESL teachers in private schools. These school teach toddlers during the day, and students use them for cram school situations. Mostly customers are student aged, to help them with their school studies outside of class time. There are also adult classes, where people might learn it for fun, or for work (outside of work/school hours). There still seem to be many jobs available for this kind of work, since there is always a demand for 'native' English lessons, and almost all of these private schools deliberately have at least one or two native English teachers for an 'all-English' class option for students (better pronunciation, mannerisms, cultural understanding etc).

The problem is getting the job- there are a lot more options if you already live in Japan and can easily go to an interview, but most who want to go work in Japan don't want to leave if they don't already have a job confirmed. Online applications are always there, but the competition is fierce like you wouldn't believe. Some larger companies recruit worldwide, and hold monthly or yearly interviews in several native speaking countries. AEON is a really big one, and in my experience is the most accessible (has interviews in a long list of cities countries). All of these big companies have websites with interview details, so check them out. Be prepared to travel some distance to the nearest major city for the interview- it's worth it. a face to face interview is the number one best way to get a job in Japan.

Hope all that ^ helps you (if the aim of your question was to scope out job posibilities, that is).