Cannondale teases prototype downhill bike with two rear shocks

That is the general rule but there is some exceptions for “prototype” bikes. I believe the rule has some type of clause with a grace period for these bikes so that a company can try it in competition but has to change it or put it into production within a year.

The GBR track cycling team had this issue with their track bikes. There was a website where you could purchase one but wait times were unknown and the total price was ridiculous. You had to purchase all proprietary frames, forks, cranks, and stems.

There isn’t a rule against non-production bikes in downhill (see Honda’s RN-01) and even production based bikes don’t have to use production frames (Sam Hill’s Iron Horse Sunday from the same period, which used the stock Sunday’s head tube, lower shock mount and that was about it). In MTB it’s been sort of self governing, suspension bikes are expensive enough that few brands spend the money on something that will never see production.
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The rules for road, track and TT bikes are many, but the UCI rules for MTB this year fit on half a side of A4 paper. They can be summed up as no wheels larger than 700c, different sized wheels are allowed in MTB and MTB only, no motors, no drop bars or bolt on extensions other than bar ends, no metal spiked tyres, no GoPros, radios, remote controls or other non-integrated electronics allowed for safety reasons, Di2 batteries and powermeter/cycle computer head units are allowed.