There's really only one rule: every foot of climbing has to be outside. Period. No exceptions. Oh, and I guess in the current climate, I must also add no e-bikes. For obvious reasons.
Without further ado, the Million Foot Hall of Fame. In case you don't habla, SS = singlespeed, which means 1,000,000 feet were climbed on a bike with one gear. You're allowed to ride a geared bike that year (I guess), but any geared feet of climbing don't count toward your SS million.