Plugin author here!
Should be
cat.custom_fields[:suppress_category_from_latest] = true
But… these values are also cached in a class variable. The easiest way to deal with this is to save!
the category as well (see here)
So this works
Category.all.each do |cat|
cat.custom_fields[:suppress_category_from_latest] = true
cat.save_custom_fields(true)
cat.save!
end