The NEW app supports 0 custom Device Handlers.
You have migrated your account to a Samsung account which is good, but that doesn’t mean start using the new App. Continue using SmartThings Classic as you have been all along. You can view the new app, but it isn’t ready and doesn’t have all the functionality that Classic does yet. When it’s completely ready, ST will notify us. 
@JDRoberts took quite a bit of time sifting through all of the topics discussing differences between the NEW STSC “SmartThings (Samsung Connect)” and the “SmartThings Classic” apps and consolidating everything into a new topic with all of these questions (including yours on custom DTHs) answered and in some cases references to the threads where these items were discussed so you can further add/reply in those additional topics. This new topic is informational only and isn’t intended to have anyone reply to it as a question / answer session as stated in the first paragraph. If you have any questions on what the NEW app does or doesn’t do, please oeruse through the following and most likely you will find your answer. It will continue to grow as new items are identified.