How can I disassociate a file extension from any default app?

There is no way to dissociate an extension without resetting (or editing) the LaunchServices database.

Apple does not provide a way to dissociate an app from an extension. All you can do is associate the extension with another app.

ibcoleman wrote:

I'm curious if anyone helpful ever offered a useful answer to this one.

https://discussions.apple.com/thread/252224212

As I said in my not so helpful way in the linked post, it's not possible.

ibcoleman wrote:

I'm curious if anyone helpful ever offered a useful answer to this one.

https://discussions.apple.com/thread/252224212

Be more specific—what file extension, what app ?