The plugin was merged upstream and should be available in v21.08.0+ (see https://invent.kde.org/utilities/konsole/-/merge_requests/360). It doesn't have any dependencies but has to be enabled with -DENABLE_PLUGIN_SSHMANAGER=ON.
"The SSH manager is by no means usable yet, but I think I got half what I want." I'm not sure we want that in a package that is supposed to be stabilised at 21.08.3...
It's quite usable on my end, at least after https://github.com/KDE/konsole/commit/ee7f71142fedd6eda49b4246e65f434e4b3c5bd5, though not necessarily preferable to manually opening an SSH session. How about, at least, for testing in the live ebuilds in KDE overlay?
The plugin is default enabled in konsole 21.12.
*** Bug 834914 has been marked as a duplicate of this bug. ***