THIS DOES NOT WORK, according to Frank, an 18-year Apple Employee Pro App specialist. He added the Pro Apps never worked that way. Yet he could not point me to anything in writing at Apple to confirm his point.
He said, "Pro Apps (including FCPX) available for download are only the current version UNLESS you purchase them at a time when the old version was available." i.e., it is impossible to download a copy of FCP 10.4x for Mojave unless you purchased FCP 10.4 when it was available. If you purchase FCP X 10.5.1, like I just did, that is the oldest version you will ever be able to download.
In other words, for example, he confirmed:
Booted into 11.2 Big Sur -- I purchased Final Cut Pro X today, 02-17-2021, from the App Store app, purchased it for $299, downloaded Final Cut Pro.app Version 10.5.1 installer at 3.68GB.
I then booted into Mojave 10.14.6, opened App Store> Account> Purchased and clicked on the confirmed purchase download.
At that time I got rotating gray wheel progress bar with an empty center.
I let that run for almost three hours with no "progress."
Then I called Apple and got the disappointing news...
Here is a screenshot of what the broken installer looks like.
NOTE: The download installer dialog I got read (see the broken grey circling icon to the right of the icon):
"Download an older version of Final Cut Pro?
The Current version requires OS X 10.15.6 or later, but you can download the last compatible version."
Please let me close this by adding I could not be more disappointed in Apple right now.
My Photoshop CS6 runs in Mojave (and Snow Leopard 10.6.8 with FCP7) -- this genius implementation of technology means I constantly have to work on two machines to get any level of production going In FCP X.
[Personal Information Edited by Moderator]