Using classic Outlook for Windows in business environments
This happened to me when I began using Office 365. In order to get Notes back, I had to do this:
To the left of "File" there is a list of shortcuts. If you hover it will say what it is: "Mail", "Calendar", etc. The box with the 4 little boxes inside is the "More Apps". If you click that one you will see "Notes" there. Right-click on it and then "Pin" should show up. Click to pin it. Notes should then appear alongside all the other shortcuts/apps. You can drag it up to bring it above other apps on the list.