Create a custom CSS containing the following script:

#suiteBarDelta:before{
content: "WARNING: This site is only for internal users";
color:white;
font-size: 14px;
font-weight: bold;
width: 100%;
}
#suiteBarDelta{text-align: center;background-color:orange;}

Here is how to enable and upload custom CSS to Alternate CSS URL.
https://www.bomzan.com/2018/02/08/sharepoint-2016-disable-newsfeedonedrivesites-from-the-in-the-ribbon/