Well, for me the end was somewhere 2 years ago. Each day since I am grateful I found Hubitat. The only thing I have left on ST is a fridge (which will stay) and 12 virtual temperature sensors with my own custom DH. These sensors (and the app that remotely updates them) will probably stop working end of this year or shortly afterwards. Only reason I kept them this long, for reasons I don't know, ST is the only platform I am able to integrate these devices with Google Home and ask, "Hey Google, what is the office humidity?". Aside from that, ST can drop off the edge of the world for all I care. I already found an alternative work-around for temperature using HA.
So, what about everyone else? Do you still have custom ST devices/apps and if so what are you going to do with them?
One TV, kitchen refrigerator, dishwasher, and range are all ST. For those kinds of devices I still like the clean interface. I have the info coming into HE with an API integration which is not relying on groovy on the ST side. That being said, I agree, SmartThings could go away and I would not bat an eye. It's not that I hate it, I just don't have an actual need for it anymore. Once you become aware of and get comfortable with the alternative it becomes mind boggling why anyone would cling to it, as I have personally experienced some still doing.
There is a community maintained Google Home integration that should allow you to integrate these temp sensors with HE. I looked into it myself but decided it was more work than I was willing to do to get my temp sensors integrated into HE so I just set up a dashboard.
I’m going to lose the ability for my Logitech Harmony remotes to control my theater lights. Big bummer. Probably going to have to hack it somehow with Phillips Hue lights or something else. Wish Logitech had added other integrations before giving up on their Harmony products.
Really only have it around for very occasional FW updates on devices that HE doesn't do (just updated a Zigbee thermostat) and because of the Ring integration that it had/has, which ironically I really don't really use any more. Maybe the real reason I still have it is I'm too lazy to do anything about it.
These (simple) sensors are already part of Hubitat. I use the same app on Hubitat and ST to update each sensor. The problem is that these identical sensors/DH on Hubitat wil not link Google using the Hubitat Google Home app. I can link the ST app and they work ok. I can link HA, which has ST integration and they work awesome.
You could tie your sensors to HE with your custom handler as you have now. Then mirror them as virtual sensors over to ST. Then share those virtuals on ST with Google Home.
I'm just looking for use cases where Mira can help.
If you try it, let me know if there is a use case that Mira doesn't provide as a sensor type. (like just RH without temperature). I'm trying to cover the most common patterns of sensors that HE users tend to have.
Can you describe how your project would resolve the loss of control over lights from the Harmony remote? Please pretend I'm a little slow and make it simple for me.
Your lights are controlled (or can be moved over) by Hubitat
Mira can "mirror" your HE lights back over to ST as virtual lights. Use ST routines to bind your Harmony buttons to actions to turn on/off the virtual lights, which will forward the changes onto HE.
I thought this was what was dying w/the end of groovy...so the part of the Harmony integration from @ogiewon that relies on ST will no longer work. Maybe I misunderstood? @JustinL - that's what you're referring to, right?
Wish Logitech had added other integrations before giving up on their Harmony products.
Maybe the real reason I still have it is I'm too lazy to do anything about it. 
