In the upper right there's a "login with password" link. I logged in successfully using my old number there just now.

Interesting. Fairly sure I remember my password from back then, but that doesn't work.

I get a "you cannot recover the password", maybe my account was too unused for too long

It has just been updated a lot between then and now. While ICQ died out in the west, it has consistently been the largest IM app in Russia and parts of Eastern Europe.