Has Metatron told a channeler that half of all energy workers in the USA will perish within 10 years because they are working with the demons pretending to be something else?

This again may well happen, and not because they are singled out and targeted for destruction. Most people are passive and not participating in the enlightenment of human. While they may not be engaged directly with the darkness to corrupt others, as is true of most lightworkers, they nonetheless have a karmic tie to the destruction planned by the darkness for all, and will therefore be more vulnerable to its effects. They will not even be in a position to ask for safety in an effective fashion, and this will be true of the spiritual community who are using a do-it-yourself approach. Their reach will not be high enough to save them, and so they will be at risk.