I want that when I click on the “Disable client files upload” button, the Client Files section remains viewable by client. However, the client cannot upload documents, modify or delete them.
In the client portal your clients have two separate tabs for files: the client files they share with you and the accountant files which are the preparer files you share with your client. Clients can delete their own file uploads, but they cannot delete, add or edit files you have shared with them in the preparer files section of your engagement view. You can use the preparer files section instead of or in addition to the option of disabling the client file upload.
Another option is to set the engagement status to Completed. That will leave the client files tab viewable, but will prevent any new files from being added. They will still be able to delete files they have previously shared.
As an aside, we added a separate category for product suggestions where community members can vote up suggestions so we can gauge relative importance and impact. I have moved your post there.