Union mount doesn't work on macOS Mojave

Is union mounting still available on macOS Mojave 10.14.5. I have tried:

• https://aplawrence.com/foo-mac/union-mounts.html

and using mount -t hfs -o union with two HFS+ images, and I can't seem to get the files in the lower mount to show up.

It' sometimes claimed that it is 'sort of' still there, and can be used, mostly untested and wobbly/semi-reliably with proper unix tools, like (gnu)ls from coreutils, but it will not be of much use if one wants to then use Finder with it.

This was answered here:

Union mounts are a very powerful feature that solves many needs, but, sadly, it's not something you can rely on for critical-mission projects, because it seems to be very little tested, if at all.

Yea. Short answer: DON'T.

Long answer: Mac OS does not support union mounts. They're there, they are honored by posix programs, but everything that using the apple file system interface bypasses it.

Which means that just about anything based on cocoa framekworks ... optimizes and bypasses union mount visibility.

I ran into this back on 10.7, and complained, only to get basically this back from apple support engineers.

— Re: Experiences with union mounts in MacOS?

Also of interest: What happened to Union Mounts? Another silently deprecated feature?