I’m sorry about support giving a confusing answer. Custom DTHs are still supported in the new app - they need to be updated to use the capabilities-based architecture that the new app uses. Basically you need to make sure it’s using standard capabilities or create the custom capabilities you need and know that it will disregard the tiles UX that may or may not be part of an existing DTH and will display the device using UI metadata. This guide gives you a rundown of how to do all of this: Custom Capability and CLI Developer Preview.
We’re waiting on a hotfix to the device plugin later this week which will get all of this working properly and out of the open beta it’s been in.