A family of Microsoft products that enable users to capture, organize, and reuse notes electronically.
To be a bit more precise:
The option to download OneNote templates is obviously still there (even in OneNote 2016) and correctly leads to the MS office template collection.
Actually Microsoft seems to have removed all OneNote templates from that directory. Honestly, in my opinion that is not much of a loss though. Those templates were almost all labeled "OneNote 2007" (which does not really matter because the format has remained the same) and were not that useful at all. Just a few fancy graphics and ornaments in the corner and some short tables or field names. Nothing which you could not create by yourself in two minutes.