Detecting extensions using web accessible resources is not possible on Firefox as Firefox extension ID’s are unique for every browser instance. Therefore the URL of the extension resources cannot be known by third parties.

and also for Chrome:

in manifest v3 extensions will be able to enable ‘use_dynamic_url’ option, which will change the resource URL for each session (browser restart). This will render this detection method unusable.

Though it should be noted that this method isn’t the only way to detect extensions.