Following enhancement was made at May 7th, 2021.

Collaboration Allowlist Enhancement
https://support.box.com/hc/en-us/articles/1500008585062

Customer are setting collaboration for internal only by folder setting.
In this state, shared link are used to share files with external users.

But with the update above, we can no longer send shared links by email from Box Web UI.
Copying the shared link and sending a separate email is a workaround, but it doesn't get users to understand.
A growing number of users are confused by the clumsy error messages that appear when sending email, and some mistakenly believe that shared link wasn't created properly.

We think that the permissions for collaboration settings and shared links are not exactly the same.
So we request to rollback this update to share items instantly via email.
Or, we'd like to be able to change settings to send email.