We always mention ATGATT religiously, and with touching a knee down, it applies. The main point with touching a knee down is having something between your knee and the ground surface. Most one-piece riding suits have the option of a knee puck coming on them, and most two-piece suits have the hook-and-loop section ready to attach. Of course, a one-piece suit isn’t required, so remember that you want the puck regardless of the suit you’re wearing.
Beyond the suit, the type of motorcycle you’re riding can significantly impact what you can easily accomplish. For instance, a sports-type motorcycle typically has higher footpegs and a sporty seating position to allow you to adjust your position to extend a knee out. Something like a cruiser, with low footpegs and a more relaxed riding position, may drag the footpegs before you lean the motorcycle over enough to drag a knee.
You can learn to overcome the challenges of dragging a knee on a cruiser, but getting there will be tough. The middle ground may be a sport-touring motorcycle. You can drag a knee easier than on a cruiser, but it may be more complicated than a true sportbike.
The motorcycle type is also just a component of dragging a knee with confidence. The suspension, ground clearance, and tires on a motorcycle can help or make it challenging to drag a knee.
The motorcycle suspension needs to provide a balance when leaning over at speed. Cruisers and touring motorcycles typically have a softer suspension that may not provide excellent rider feedback when leaning over.
Ground clearance must be a concern because you don’t want to drag a footpeg, kickstand, or exhaust on the ground when leaning over. Anything touching the ground may relieve some of the traction the tires provide and cause them to lose grip. And mentioning the tires, a hard-touring tire may not hold well at speed. In contrast, a grippy sports tire will provide much traction when leaned over with a minor contact patch touching the ground.