Hi. Apparently this is a recent update. There are now link visibility permissions required to share previews of a survey project, including when you share an invitation or reminder you have set up to a contact list, etc. This is really the only way I have traditionally shared previews, mainly to make certain the invitation and reminder were properly set up, in addition to using the preview myself to make certain the project was set up correct.
The default setting for preview link permissions is brand internal, so anyone the preview is shared with will be required to log in in order to access the previwe link. This setting can be changed to public. The project needs to be previewed from the builder, then the link visibility permissions can be changed under the Share Preview tab.
I discovered this completely by accident. I had a couple of chat sessions with Qualtrics support and learned that this was a relatively recent change and not really promoted or publicized to users or brand managers.