kde-base/okular-4.8.2 does not require app-text/poppler built with lcms support. I just recompiled poppler without lcms and threw several PDFs at okular - they all worked the same as usual. Then I recompiled okular, everything was the same as before. I cannot find any references towards okular actually needing lcms, it was merely put into the dependencies as a workaround for the badly handled bug #284783. I have not tested okular versions < 4.8.2. So I am hereby asking to drop the lcms requirement from the ebuild. Thanks for your work!
Indeed, you're right. While getRGBProfile and other function are actually exported to API when USE_CMS is defined (at poppler compilation time), okular never used them.