Moreover, until early 2018 YouTube provided a dedicated tool for creators to share other channels content to your own, implying that sharing of other channel's content is nothing to worry about. 

YouTube never communicated to us that reusing content from other channels is an issue. The channel is in good standing, thus there were no warning signs at all, until when on January 31th 2019, YouTube suddenly (again with out notification) disabled monetization for us.