Your question has no actual question it it, or I am completely missing it.
I assume "nothing happened" because LibreOffice still works for you? Try removing boost-compat and see if LibreOffice still works then.

'Still?' Never had it run in Slackware-current. After installing that lately, I built dependencies & Libre Office binaries from source code. It installed. That version didn't run; neither did yours (after upgrading) (when launched in KDE5, icon/taskbar thing starts as if it'll start, then disappears.) I realize I should've said more, but even 'soffice' in xterm does nothing: no output of what it's doing to use for debugging (even for five or 10 minutes... though unlike from KDE start menu, doesn't exit immediately; I have to ^C to return to prompt in xterm.)

Of course, sbopkg lets you build/install either from source code or a prebuilt binary, and I have no idea which yours is like (of course sometimes being different) or dependencies are (for any of your packages.)

The fact that the official LIbreOffice binaries won't run, nor my Slackware-compiled packages, nor the ones you compiled yourself... it must be something with your local OS installation (missing/outdated/co-installed software) or your particular PC hardware possibly combined with graphics drivers.

The libreoffice packages in my repository are compiled on Slackware, by me.
For all my packages, there's a SlackBuild script available which has build-time and run-time dependendies listed in the script's header part. And there's a slack-required file with run-time dependencies as well which will get embedded into the package so that tools like slapt-get can do their own dependency resolving.

That's the only version I didn't install after boost-compat (don't want to build entire Java SDK before many other things to get back to normal) Likely you're right about problems, but office/word-processing software isn't a top requirement to me (when is on other PCs) as much things used more (compilers, etc) when GUIs were not OSes. So people could give me more advice, or I can just wait for the stable version.

What could be an issue is any problem that might mess up other things or prevent me from testing things that other Slackware users probably also consider more important... all kinds of stuff seems to be compiling (or being interpreted) fine though...

That happens here too, it is a matter of minimizing and restoring the window.
I don't know if this happens with full KDE5/Plasma installed, I have only stock -current
qt5 files installed.

It happens only with qt5 uncommented in /etc/profile.d/libreoffice.sh. I still preffer
running LibreOffice with qt5 than with gtk3 with those enormous text boxes and buttons.