Description of problem: Now that new compilers are available in SCL, I think this software could be packaged for EPEL7 too. I need it for nextcloud-client 2.5.x
Will likely require a not-so-insignificant amount of work, if it's possible at all (now RHEL may have a too-old Qt 5 to support recent qtwebengine).
Looks like EL7 has Qt 5.9.2. I'd expect at least QtWebEngine 5.11.x to build against that (latest LTS release at the time), hopefully also 5.12.x. Though of course I haven't tried it. The other issue will be other system library dependencies: I think you would have to enable a lot more bundled libraries than in Fedora because the system libraries are just too old.
qt5-qtwebengine will not be built in epel7. The amount of work to built it at this point is too big.
I doubt QtWebEngine 5.15 would build at all on EL7, considering that it is stuck on Qt 5.9. As I wrote 3 years ago, maybe someone can get QtWebEngine 5.12.x LTS to build. But that's an ancient Chromium with at this point only sporadic security backports from Qt. (The latest so far was 5.12.12 on 2021-11-25.) And it would be a full-time job for a week or two, I guess. And I think there are only a handful people in all of Fedora who could pull it off at all. As one of those people, let me state that I really do not have the time to work on backporting QtWebEngine to an ancient RHEL at this point. (I have a VPS running CentOS 7, but I do not need QtWebEngine on it. My desktop and notebook run Fedora.) And seeing how nothing has happened for 3 years, I do not see anybody else who is both interested *and* able to do it. But if someone wants to try, as long as EPEL still accepts new packages for EL7, you can always have a try at it. Just do not expect Troy or Rex or me to do the work for you. (So I agree with the WONTFIX resolution.)