Back in 2018, Microsoft announced that OneNote for Windows 10 will replace OneNote 2016 as the default OneNote experience for both Office 365 and Office 2019. Microsoft even rolled out OneNote UWP app to many Office 2019 users.
At Ignite 2019, Microsoft surprised everyone by extending the mainstream support for OneNote 2016. Microsoft basically asks enterprises users who prefer the old Win32 app to continue using it.
New support dates for OneNote 2016 now align with Office 2019 (October 10, 2023 for mainstream support and October 14, 2025 for extended support).
Microsoft also announced that Dark Mode for OneNote is rolling out to Office 365 subscribers. Microsoft is also planning to bring new collaboration features like @mentions for quickly pulling collaborators into your OneNote notebooks.