[jaffathecake]

It's ok, but you're still being taken out of the flow. Especially as the notes appear arbitrarily on the left or right. And yeah, you have to find your place again afterwards. It's way better than scrolling solutions though.

That particular example on mobile falls back to a revealing pattern, which is pretty good.

It still has the problem where it's just a test of your curiosity. You don't really know what the supplementary content covers until you expand it. The link text is just a superscript number which is kinda useless.

This is why I prefer the solutions in the article where the supplementary content has a heading that hints at the content.

[account42]

The entire point of footnotes is that they have been moved out of the main text because their presence would negatively impact the flow. If most readers are expected to want to read them then they should just be part of the text and probably not even in parentheses.

[jaffathecake]

Right, but it should be easy for the reader to decide whether to divert to the additional content. A superscript number tells you nothing about the additional content. Whereas on the web, link text or a details summary can help you make a decision.

[rendaw]

Instead of using numbers, what about having a line from the text to the sidenote?