The new version of the app I'm working on has an App Clip but I cannot see the Clip (nor in messages nor in Safari smart banner):
App is not installed in the device I'm testing
App is live (on gradual release, now at 5%)
App site association is updated with the appclips part
Xcode was used to create the Clip target, so the capability was added
Universal links were in place before, so I just added a new item for the appclips to the associated domains (appclips:example.com)
I created an advance experience to make explicit which url can invoke the clip: https://example.com/supported/*