The advanced user app where you first posted is a community built utility.
The Rules API is an official SmartThings feature. So I’ve moved your post over to the forum section about the rules API, where hopefully you’ll get more responses.
OK, that said, I think the short answer is that SmartThings rarely tells us in advance what their roadmap is, so nobody here knows about the future of the rules API. My guess is that The public facing features will eventually be deprecated just because they seem to be going to a “simple as possible“ model, which does meet the needs of the vast majority of their users.
Power users do have quite a few other options, though. If you’re willing to set up a separate server device, MQTT is a possibility or you can use node red as The following project report discusses:
So if you want to protect your time investment, I’d go one of those two routes rather than the rules API. But that’s just me. 
Yet another alternative is to use Webcore on hubitat and then tie it back to SmartThings with one of the community-built integrations between the two platforms. That would require less programming skills than node red or MQTT, so some people might prefer it. Especially if you already know Webcore.
The following thread gives an overview of the various options right now:
Replace Groovy with Automations—what’s your plan?
Choice is good. 