In addition to
I’ve noticed the same thing: soft proof set to sRGB reports no out-of-gamut pixels while gamut check does. If I understand correctly they should match, don’t they? The manual is quite unclear on this, stating only that those are mutually exclusive. And, the same as @nwinspeare has shown, I cannot see any clipping or loose of details in areas shown as out-of-gamut. I suspect that gamut check may probably lie, or it might be my error somewhere…
I will defer to more experienced users that answer after me, but DT offers many choices for soft proofing profiles I would expect that you need to set the softproof profile to match the intended output profile, which in your case would be sRGB. I just checked this change on my DT and the gamut check for sRGB was different to rec2020. I have my working profile set to linear REC2020 RGB, but softproofing to sRGB.
I’m a bit hopeless on colour management, but as I understand it, softproofing doesn’t indicate anything, just shows what the image will look like when exported or whatever into the colorspace selected.
Gamut check, on the other hand just highlights the pixels that don’t fit into the chosen color space.
Waiting to be corrected… 
Per the user manual:
gamut check - “This function highlights, in cyan, all pixels that are out of gamut with respect to the selected softproof profile.”
soft proof - “This allows you to preview your image rendered using a printer profile to see how colors will end up on the final print.”
So the way I read it, gamut check tells you which colors will and won’t make it to the printer color space and soft proof shows you how the ones that made it will look after translation.
Also waiting to be corrected… 
Softproof intent is for you to load the printer profile so you can “see” how the image will look on the paper. DryCreekPhoto.com used to provide printer profiles for free to local printers (eg. Costco, Cord), but they stopped are only doing it as a service. You can still access the database and download one so you can test the difference in darktable.
Whitewall has the profiles for their printers/papers. https://www.whitewall.com/us/photo-prints/ultra-hd
I could not find the webpage to download for Bay Photo.
In summary, see if your lab has profiles for the papers/inks they use.
Oh! I see, my misunderstanding. So soft-proof doesn’t mark anything, just showing how it might look like, given that display profile is ultimately wider then the printer one)))
Thanks a lot! However I doubt I could be able to make use of it, because my printer
(Epson L8180/ET-8550) unfortunately only accepts sRGB 8-bit input (at least under Linux) and does all the magic inside, however I suspect it’s potentially more capable ink-wise. That’s another large topic I’d like to discuss. I found three ways to connect the printer to the PC (let alone Ethernet vs USB thing), none of it is fully satisfiable.
Correct.
I found a last year discussion on this topic: Very confused with color management and profiles
I think I’ve enlightened enough for the moment.
Thank you all!