There was/is a flatpak of 2.8, but not with separate+, though you should be able to add seperate+ as a plugin.
Hey @prokoudine, is there any thing in the short term planned to have separate or a similiar alternative? We have downgraded to 2.8 for now.
What needs to happen is that the separate+ code (or, rather, separate- code – the simplified version of the whole thing) has to be merged into respective TIFF and JPEG plugins of GIMP. So far noone has volunteered to do this.
See support for exporting CMYK TIFF format (#356) · Issues · GNOME / GIMP · GitLab for details.
It is possible, decomposing and using separate+ see: https://youtu.be/5z98GezQVtI 3 minutes
However. It is not worth the effort. You need to see any changes and at the moment Krita can be used.
Lack of CMYK is still used as a reason to knock gimp.
In the 86 (largely irrelevant) comments here: https://www.theregister.co.uk/2018/08/22/gimp_2106/
but as Prokoudine comments, needs one of you clever guys to write some code.
Been using CMYKtool for ages, but on my Win10 I cant make it work.
but just found:
Cyan Prepress
It can only export to tiff, but thats workable for me when using Scribus as DTP.
Though you have to install some cmyk icc profiles, which you can google “adobe icc profile end user”
Yes, it seems to be working great in Win10, I’m very happy to say (just found out myself - me taking a pic of me, by the way ;))
I know I should have opened some of the files and THen taken pics, but was a bit tired at the time, but on win10, as long as you have it set up right (you know, with plug-ins) it works great 
I find this solution: https://e-smog.org/blog/gimp/2018/10/10/separate-plus-gimp-2-10-6.html
The problem was in 32-bit version of old plugin.