I remember my ICQ number, not used since 2001 I remember my compuserve number too -- 101611,1220 - not used since 1996. I remember our first phone number as a kid, 818641, which we changed in the early 90s too. I recall friends numbers too, and a bank account I had from 1993 to 2004.

However my slashdot number, which I've had nearly 20 years, I know nothing more than it begins with 2.

The modern numbers I remember are my mobile phone number, my wife's, and my passport numbers (phone numbers as we've had them over a decade and all of them because I have to write them on forms so much). The only other numbers that spring to mind are my staff number at work (used in various forms, had since 2003) and my bank numbers (needed to log on)

If you use a number a lot, you learn it. If you don't (like usernames which are saved) you forget it. I can barely remember my credit card pin as I use contactless so much, but muscle memory seems to work there.