As you may have seen, Rafael Rivera revealed that File Explorer in the latest Windows Insider Program Dev channel build secretly supports tabs. Here’s how to enable this feature.
First, and most obviously, you must enroll your PC in the Dev channel of the Windows Insider Program (Settings > Windows Update > Windows Insider Program). And you need to be using the latest build, which at this time is build 22472.1.
Then, download and extract ViveTool. (I did so on my desktop.) There are two files.
Next, open a Terminal with administrative privileges, navigate to the desktop (or wherever you extracted those files) and use the following command:
./vivetool addconfig 34370472 2
Then, reboot. Voila! Tabs are available in File Explorer.
If you wish to remove this feature, use Vivetool again, but with this command:
./vivetool delconfig 34370472 2
You will have to reboot again, of course.
Thanks to Rafael Rivera for discovering this, and for the steps needed to enable it.