A few days ago an update to my Office 365 Pro Plus removed OneNote 2016 from my company laptop. If you want it back you can bring it back using the installer available in the article "OneNote 2016 is missing after installing Office 2019 or Office 365". Notice however that OneNote 2016 is no longer being updated with new features as stated in the article "What's the difference between OneNote and OneNote 2016?".
Reasons for installing OneNote 2016 are, for example, having local-only notebooks (OneNote for Windows 10 requires notebooks to be on-cloud) or being on older client operating systems such as Windows 7 or Windows 8 or using COM add-ins such as Onetastic for OneNote.