SmartThings is in the middle of a transition to a completely new architecture, part of which is called Edge.
The Samsung-hosted free groovy cloud will go away. For hub-connected devices, including Zwave and Zigbee, the DTH replacement will be Edge Drivers, which are written in the Lua programming language and will run locally on your own hub.
Here are the official announcements:
Edge is still in beta and they missed their target transition date from the first announcement so no official timeline details at the moment but they are starting to roll out some Edge Drivers.
You can find a list of community-created Edge Drivers in the quick browse lists in the community-created wiki. Just remember that as of January 2022, everything is still in Beta.
To use a custom Edge Driver, you have to subscribe to the author’s channel, which means following a weblink they will give you. It can then take a little time for the driver to be downloaded to your own hub (in some cases as much as 12 hours has been reported).
Once it is on your hub, you can add a device of that model to your account by using the “scan nearby” option in the ST app and it should automatically use the driver that matches its “fingerprint.” The device should then show up in your app under “no room assigned.”
Note that the IDE will be going away when the groovy cloud goes away, and it has not been updated to deal with non-groovy code, so all the edge drivers will show up just as “placeholder“ and if you try to make changes to them through the IDE you can break communications with the device.
If you have any questions about a specific edge driver, ask in the author thread for that channel.
(And, yes, this means that all groovy DTHs, stock and custom, will stop working once the transition is complete. SmartThings says they are working on a method to automatically transition everyone if there is an Edge Driver with a matching fingerprint, but if you are using a custom DTH that might lose you some features unless you can find a custom Edge Driver with feature parity. But this is just something you’ll have to research model by model. )