And if you want to stay with a OneNote-a-like model, it's worth bearing Outline (macOS, iOS) in mind; it's OneNote-compatible and has been quietly developing away in the background for quite a few years. It's a much, much better app than it used to be, now with a proper search function.

The other, totally free and independent app worth mentioning is Growly Notes (macOS only), which is excellent; the developer is ex-Microsoft, and offers a wide range of free software on his website (https://growlybird.com Growly Notes is basically a OneNote clone, and again, has been around for quite a while.

Neither of them does something which the desktop version of OneNote does still do, namely allow you to fold indented subparagraphs.

bartb wrote:

Microsoft is sucking everything up into the Cloud (u need to use
OneDrive with OneNote) and OneNote files are in a propritary format and
the export function kind of sucks. I loved OneNote but MS is going into
a whole different direction. I'm thinking KeepIt in combination with
DEVONthink might be more to my liking. As a side note I came across the
following quote Gary Lang (Gary's Newsletter): "Basically, think of
OneNote as an OS, with OLE for live, embedded objects and tasks. People
really want this, but only with a OneNote-like front end." Indeed!

Thanks for the replies everyone!