1) SVG as a background image in style properties. This works.
2) SVG in the Header Logo Image Path property. This works.
3) SVG in an IMG tag in a post. This works.
4) SVG as an avatar. This does not work. It is not recognized as an image.
5) SVG as a post attachment. This works if you allow that extension, but it is not processed as an image (no resizing, thumbnail, or inline display).
So the only issue here is in uploading SVG files as images (avatars and attachments). GD doesn't support the format but ImageMagick does. Should XenForo bother adding it? I say no. Stay with the basic formats that are common to both image libraries. Anyone working with vector graphics knows how to convert to a different format before uploading.
To do it *right* you almost would need to redesign the avatar and attachment system. For example the system currently creates 3 sizes for each avatar (small, medium and large). With SVG, you may not want to do that.. if the image isn't totally detailed, it can be fairly small in terms of storage size even at the largest size... so if that's the case, you probably wouldn't want to have scaled down versions stored. But if it *is* a huge SVG, you may be better off just converting it to a raster version in a different format (like JPG or PNG) at all sizes. But then you kind of lose the value of SVG (being vector).
While I love the SVG format, I think the complexity involved in making SVG work *right* for avatars and attachments probably isn't worth it, but that's could just be me. Even though I use SVG extensively for site graphics, if such an option were available for avatars, I'd disable it.
Well generally no one but there are still people/businesses still running older versions of Windows and they use applications that will only work with IE8 and they have no want or desire to update said product to work with newer browsers. My wife has a job like that and they connect to a state program and the state has no plans to update that program so they are stuck in IE8.