Webflow has done a great job in implementing advanced CSS functionality in its UI. That's why it's so surprising that something as basic as bottom and right side relative positioning isn't available. For a designer, there are lots of use cases - e.g. form buttons with a right-aligned chevron/arrow. Yes there are ways around this, but they're a bit hacky and all involve some degree of compromise. I can't imagine it would be that hard to implement relative positioning from all four edges.

I've yet to run into a project in which the design doesn't call for a background to be positioned relative to the bottom or right side of an element. It's frustrating that the background positioning UI doesn't cover even the basic functionality of the background-position CSS property.

Indeed.