I'm using it on High Sierra, it works as well as it ever did.
I think the reports of the death of Aperture may be exaggerated. My understanding is that Apple has no plans to kill it off; they just can't promise that future operating systems will be backwards-compatible with Aperture. Me, I have it up and running on two MacBooks and an iMac. The latest OS I have is Sierra.
Aperture is indeed dead. No one can buy it. There are no upgrades to it and Apple has clearly stated they won't support it.
Some can use it while they it still works, but the day is coming when something will break on it.
A real shame- Aperture created the DAM software category and for a good deal of time, was market leader. It should not have been discontinued.
Obviously they couldn't make it work on a phone and that and the watch are the only products that Timmy and the gang care about anymore.
I'm not a big fan of Photos, but manage to use the Nik Collection very well from within Photos. I go: Image-Edit with; and theres is listed all of them. When you click save, the Nik program will close and the adjustments are saved within Photos. If you change your mind, you go Edit and Revert to original.
Personally, I see little point to investing more effort and work in using Aperture at all. ...
[*]Aperture has not been developed further since before 2015, and does not run at all on macOS Catalina since it is not fully 64bit.