I've had the one like you linked to on Skidim for probably 5 years now, never had a blockage issue. But I also thoroughly flushed out my manifold and risers in the recent past for other reasons, so there's likely less junk floating around in mine than others of a similar age. You could probably spin off the valve from the elbow and get a wire in there to clear it out a bit if you had to. Then you could also take it off completely every few years... better than every year for sure.
And as was discussed at length this fall, with these drains in particular I still add non-toxic antifreeze to be sure to displace or dilute any water that might be left in the manifold after draining. Removing the whole plug is a much more certain way of getting the water out, but this is a nice quick way and the AF provides another layer of protection.
1989 Bravura - 1998 GT-40 engine