Quoted Text Turning of boxes with labels sounds very limiting.

Is this embedded HTML something we should expect to increasingly see from other SVG editors, or is this a peculiarity of draw.io's implementation? How widely supported is draw.io's version of SVG+HTML in other SVG rendering environments?

draw.io uses foreign objects (FO) to embed arbitrary HTML labels within SVG shapes. It's partly to allow users a wide scope of flexibility as to what they can create within a shape, but a lot of the reason is to give us word wrapping, which is very hard in native SVG text.