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I received some letters in Japanese 10 years ago saying I was part of it, but I haven't really heard anything since

It is far far too late to get paid. Several deadlines have passed.

Wrong. Look at Self-Approved Claims here: https://claims.mtgox.com/faq

Doesn't that page say the deadline for those claims was in 2019 (not even to be submitted, but to contest it if you're refused)?

You're out of luck if you haven't archived a registration or password reset email

Also probably out of luck regardless due to deadlines