They’ve explained several times, so I’m not quite sure where the confusion is.
They are no longer providing a free Groovy cloud, and they are not allowing customers to upload custom SmartApps to their hubs.
If you want a custom smartapp, you have to write it yourself and host it yourself. Or the original author has to host it and make that available. Then these will connect to the new platform through the smartthings REST API. These are now called “endpoint smartapps.”
So far a few platform partners have done this, including Ifttt, sharptools, and actiontiles, all of which previously had custom groovy smartapps, even if you didn’t know they were there.
As far as community developers, there have only been a couple of those, but they do exist. These include one for myQ, one for lock manager, two that support integration with hubitat, and a few more. There is a section in the community-created wiki that lists the ones that have been publicly announced so far:
There’s also a list in one of the community FAQs about some of the specific replacements for some popular SmartApps.
Replace Groovy with Automations—what’s your plan?
Here’s what the new architecture looks like. As you can see, smartapps still exist, but now they come in through the REST API:
So those have already been around and working for a while, it’s just that not many people are writing them.