Hi @Nurlan_A , I and many others agree that this would be a wonderful feature for collaborating on docs with sensitive information.
The official solution (so far) is to use Cross-Doc. Cross-Doc is a bit of a pain to use (relative to the extreme ease of use of the rest of Coda) but it get’s the job done when you want to share only certain pieces of data to certain people.
Another (not very secure) option would be my answer to this question over here:
Note: the latter option is only for viewing (not editing) data on pages.