I couldn't fix the LXC problem on recent releases, so, I've decided to go back to SFOS4.2.
Some fixes (wifi, netlistener flod, etc.) were included in newer releases, therefore, I've included them into the hybris-hal for 4.2. So, if you'd like to go back to 4.2 but you want wifi fixed, please flash this hybris-boot image into /boot partition. (hybris-boot-sfos42-08-wd-fixes_usbmoded-wifi-netlistener.img)

Edit (2024-09-25):
I've found the root cause of the issue with lxc and managed to fix it.

When testing sfos4.6 on the device I did play with lxc a bit more and found out that sfos upgrades also install newer version of lxc packages (more precisely: lxc-libs, lxc-templates & lxc).
On sfos 4.2 and 4.3 the lxc version 3.0.1 were installed. However, sfos 4.4 upgrade did install lxc version 4.0.10. This was the point when the lxc-attach started crashing.
It seems my kernel has got some missing elements therefore the newer than 3.0.1 version of lxc-attach has stopped working. It aborts with the error posted here.

There were two ways to solve this issue.
1. backport all necessary kernel/device changes for supporting that memfd
2. install older version of lxc packages and try how they work on the newer sfos version

The first option would the best and comprehensive as it will ensure compatibility with newer lxc versions. However, it can be challenging and time consuming. Therefore I've chosen the second option.
I downloaded those lxc packages from older sfos version of jolla repositories and tested them.
The latest 3.0.1 version (3.0.1+git4-1.4.3) from this repo works well even on sfos 4.6.: