It depends....Intervals come in many flavors depending on the objective. The first thing you have to get your arms around is the difference between training "something" and just riding or competing. Once you decide you want a training program, you have to do things that will cause change which means you will not be doing what you want to do or are necessarily good at. You will work on your limiters and weaknesses.

So intervals z2, z3, z4, z5, z6 and z7 can be done at low, self-directed or high cadence. When training, I seldom do self-directed cadence. I am always working on improving something so self selection is off the table.

I do low cadence z4 and z5 to build strength and high cadence to build aerobic capacity and neuromuscular capability. One can always elect to spin slower but you must train to spin faster.

Friday, I did 3x6 laps at the track behind the motor in 82 gear inches at 32 mph with the goal of improving cadence. Earlier this year, I was doing a lot of low cadence z4 and z5 intervals to increase strength and muscle recruitment.

The hardest intervals for me are the high cadence 100 rpm or higher 20 minute z4 intervals. High cadence at high power is very tough and it takes a long time to master. It is also the most desirable and useful. YMMV