When creating docs only meant to be consumed online, the page breaks have gotten in my way before many times. Splitting up paragraphs because they don't fit on a page etc.

So I can see this change having a big effect on consumers. If by "how big a change" you meant "would anyone even care", I think people will care, yes. Including me.

How big a change was it to implement? I don't know.

Note in addition to not having page breaks, it appears to have several "responsive" features added too (from the OP description, I haven't played with it yet myself). Lines wrap at whatever your screen size is (including zoom level), and there is apparently some screen-size-responsiveness to at least some images too.

I couldn't say how difficult this was to implement, having no idea what the code is like, and knowing that large legacy codebases can make naive predictions of how difficult a given change might be unreliable.