Why doesn’t assoc work here to disassociate a file type association?

I associated a file extension with an app (.txt2 with notepad) by creating a file and naming it a.txt2, clicking it, and selecting notepad as its default app.

Then I tried to disassociate it from notepad using: assoc .txt2= from an elevated CMD window. [assoc docs]

It changes nothing. Clicking the file still opens it in notepad even after restarting the computer.

Why? Is there some step missing here?

More info

Typing assoc .txt2 to check its associations returns:

File association not found for extension .txt2

Answer

The assoc command is very old, dating from DOS, and does not use well the
newer file-association mechanism of Windows.

The proof is that after running assoc .txt2=notepad,
double-click on a .txt2 file does not launch notepad,
so you need to still specify the default app.
The opposite is also true: Running assoc .txt2= is not enough to undo
the newer mechanism.

Searching the registry finds that the command assoc .txt2=notepad only sets
the registry keys HKLMSOFTWAREClasses.txt2 and HKCR.txt2, which are
deleted by assoc .txt2=.

However, setting the default association through Explorer with “always use”
creates keys chiefly under HKCUSoftwareClasses.txt2
and some others.

This is why assoc .txt2= cannot undo the assignment through Explorer,
as it doesn’t know about these new registry keys.