How to Hide the [New] Button in Lookup Search Fields in Lightning Experience

To prevent the [New] button from appearing, you must remove the "Create" permission for the target object. This prevents users from creating new records via the lookup search.

Steps:

1. Click the gear icon in the upper right and select [Setup].

2. Navigate to [Administration] > [Users] > [Users].

3. Select the target user and click their assigned [Profile] name.

4. In the [Object Permissions] section, uncheck the [Create] permission for the target object and click [Save]. Note: If a Permission Set is assigned, ensure the permission is also removed from the Permission Set.

A request for this feature has been posted on the Salesforce IdeaExchange. If you would like to see this feature implemented, please vote (Upvote) using the link below: