I've had a lot of users asking what happened to the Send to OneNote button in Outlook. Others just wanted to know what happened to OneNote.
An upgrade happened and OneNote 2016 is no longer included with the Office 365 or Office 2018 installations. Microsoft is encouraging users to use the Windows 10 OneNote app instead. However, if you want to use the desktop version, you can download it at http://www.onenote.com/download. If you use 32-bit Office, click the Windows Desktop link. If you use 64-bit Office, click Other download options to see the link for 64-bit OneNote.
If you have the "Store" install of Office, which comes preinstalled on new computers, installing OneNote 2016 will remove the store version and replace it with the click to run version. If you use Outlook with an IMAP account, export the 'this computer only' folders before installing OneNote 2016 as it will delete your Outlook profile and data files.
When OneNote 2016 is installed, the Send to OneNote button in the overflow on the new Simplified ribbon.
It's in the usual position on the classic ribbon:
If you install OneNote 2016 and it is saying it's unlicensed, see
OneNote 2016 is missing after installing Office 2019 or Office 365
Also see Frequently Asked Questions about OneNote in Office 2019