What does this mean? This is a pure HTML solution, not just "technically" but in reality. (And before iframe there were frames and frameset). Just because the author doesn't like them don't make them non-existent.
An iframe is a window into another webpage, and is bounded as such both visually and in terms of DOM interfaces. A simple example would be that an iframe header can't have drop-down menus that overlap content from the page hosting it.
They are categorically not the same DX/UX as SSI et al. and it's absolutely bizarre to me that there's so many comments making this complaint.
They would be a lot more useful if we could write e.g. <iframe src=abc.html height=auto width=100> so the height of the iframe element is set by the abc.html document instead of the parent document.
Then you need a postMessage to send body size to parent frame which then needs to listen for messages and resize the iframe element.