The IOUtils Migration Guide is lovely. But it isn't the easiest to change when (for example) you notice that there is now indeed a IOUtils.exists method.

Let's move it to the sourcedocs, and replace the gdoc with a redirection notice to the new location, shall we?

A public gdoc's not bad, but source docs is better.
This place was chosen because the implementation is nearby,
even though it's really support for toolkit and browser stuff.

https://hg.mozilla.org/mozilla-central/rev/1044b04cf89b

I think this section should be edited to make the examples actually analogous. OS.File version says:

await OS.File.removeDir(path, { ignoreAbsent: true });

but IOUtils version says:

await IOUtils.remove(path, { recursive: true });

But this method does support ignoreAbsent. The IOUtils version should say:

await IOUtils.remove(path, { ignoreAbsent: true });

The difference is that OS.File.removeDir doesn't have a recursive property, and conversely, it has an ignorePermissions property that IOUtils.remove doesn't support. Maybe an options section for this method should be listed.

I don't actually know the code well enough to comment, so I've filed bug 1743412 for the discussion and for tracking the resulting work.