IMHO Plasmate should not be released for default build also with Plasma 5.10.
Distribution packagers seems to just look if things built, but not test the functionality, at least when it comes to Plasmate.
E.g. for openSUSE plasmate is part of the package plasma-sdk. And when people install that, because they want to do some plasmoid development, then see plasmate and try it, when sitting next to them you have the non-joy to possibly learn new non-quality descriptions.

And given the visible relationship to KDevelop, there is cross-attribution to KDevelop, not nice.

I know there has been much love and dedication put in Plasmate, and it would be awesome if somebody would continue that, to give users some comfortable way to add own extensions.
But in the current state it sadly achieves the opposite, people get frustrated because of the bad experience and lost time.h

There is a new git repo intended for this, kde:plasmate, I'll ask on mailing list if it's better to use that or just disable

rather than disable the build I've put it into a separate repo kde:plasmate and removed it from kde:plasma-sdk both Plasma/5.10 and master branches