OneNote for Windows 10 and Onetastic
Update: On November 4th 2019, Microsoft announced that it will extend mainstream support for OneNote 2016, reversing their earlier decision to stop adding new features to OneNote 2016. This means you can continue to use OneNote 2016 and enjoy the benefits of Onetastic for the foreseeable future. OneNote 2016's support dates are being aligned to that of Office 2019. Office 365 subscriptions are being also updates to include OneNote 2016 by default in the near future.
You can find the announcement at: https://techcommunity.microsoft.com/t5/Office-365-Blog/Your-OneNote/ba-p/954922?sfns=mo
There is also an FAQ about the announcement available at: https://support.office.com/en-us/article/frequently-asked-questions-about-onenote-6582c7ae-2ec6-408d-8b7a-3ed71a3c2103?ui=en-US&rs=en-US&ad=US#gen01
The original announcement by Microsoft can be found here. Below is a part of the announcement regarding the support policy for OneNote 2016:
What’s happening to OneNote 2016?
While we’re no longer adding new features to OneNote 2016, it’ll still be there if you need it. OneNote 2016 is optionally available for anyone with Office 365 or Office 2019, but it will no longer be installed by default. If you currently use OneNote 2016, you won’t notice any changes when you update to Office 2019. We’ll continue to offer support, bug fixes, and security updates for OneNote 2016 for the duration of the Office 2016 support lifecycle, which runs through October 2020 for mainstream support and October 2025 for extended support.
Microsoft created a page with some frequently asked questions about this topic which you can find here.
OneNote Windows 10 app currently does not support add-ins. So we cannot develop Onetastic for it. OneNote 2016 will still be supported till 2025. It just won't get new features. We will continue to develop and support Onetastic as long as OneNote 2016 is supported by Microsoft.
So if you depend on Onetastic on OneNote 2016, you will be able to continue to use it for several years. In fact there are many Onetastic users who are still using OneNote 2013 or OneNote 2010. If you would like to transition to OneNote app for Windows 10 now, unfortunately you won't be able to run Onetastic on it. If Microsoft adds add-in support for Win 10 app it may be possible for us to develop Onetastic for it.
If you for some reason don't have OneNote 2016 installed on your computer but you are looking for a way to get it back, you can downloaded it using the instructions here.