Just to follow-up. I went ahead and got a DZ6HD. I can confirm that this DTH works in the new smartthings app. Yes, you don’t get the custom UI that has some handy preset dimming buttons. However, the device will still function without issue in the new app.
Also for the firmware version DZ6HD, that I have (1.7), the default handler does not show the bug related to remembering what the last dim level is at.
The custom DTH, however ([RELEASE] Leviton Decora Z-Wave Plus Dimmers (DZ6HD & DZ1KD)) allows you to tweak settings that you can’t tweak any other way (like fade in and out times). So, if you have this dimmer you will want this DTH anyway. There is no down side that I can see.