Manual mounting works with any device.
I have to understand why udisksctl fails.

Well, you're on your own there with udiskctl. Thanks for stating the problem clearly. I would see if the udisks program is doing much, as udiskctl probably uses that. There's also the tangled nightmares of settings, systemd, D-bus, automount, and permissions to navigate.

Slackware Master rworkman's solution #20 worked for me! Hurrah! I still had the old mozjs52 lurking in /var/log/packages. Clicking on the thumb drive in dolphin's Removable Drives stopped working after a new-kernel reboot. Didn't notice because I had to use sneakerware after a ribbon cable on a laptop's DVD interface snapped a couple of leads. After much wailing and knashing of teeth, invoked removepkg mozjs52, cast the installpkg spell on mozjs60, which didn't upgrade mozjs50 a couple of weeks ago, and now it's time to get on with Happy New Decade! Thanks, Robby!

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The automount was restored by one of the latest updates.
Was probably between:
Fri Oct 23 18:50:26 UTC 2020
and
Mon Oct 19 20:16:19 UTC 2020

Thanks everyone btw.

I too was bitten by the mozjs issue. After the updates from a few days ago, automounting a USB drive no longer worked. I could still mount it manually as root (remember the good'ol days ?), but XFCE no longer mounted it automatically.
Even worse, after a reboot, XFCE suddenly filled my desktop with drive icons for devices like floppy, shm, pts, ...
Removing the mozjs-52 package solved the problem. Just wondering how something like Mozilla javascript is linked to mounting usb drives ?

The automount is gone again after an upgrade !!!
Nevermind, I'll reinstall everything one of these days.