Since a few hours ago the Okular version available in the Microsoft Store for Windows has the same signature support than the Linux/FreeBSD versions.
Some screenshots, though it's nothing impressive, it's the same User Interface that we already have had for a while
|A file I just signed, showing the Signature and Certificate panels|
|The PDF backend settings|
This "new feature" has been delivered with the update of the just released Okular 21.12.2.
The interesting part is that adding Signature support for Okular on Windows didn't actually require *any* change in Okular itself, its code was already good.
What we needed was to add NSS  to Craft  and also some tweaks to poppler , because sadly NSS doesn't behave exactly the same on Linux than on Windows, and once that was in, just start Okular again and voilà everything was working as expected :)
Relevant Merge Requests:
 NSS = the library poppler uses for signature support
 Craft = the meta build system we use for building KDE software on Windows
 poppler = the library that Okular uses for PDF support