IMHO, the original spec had it right when it said (paraphrased) “just upload your tw.txt file wherever”. The essence of micro-blogging, as opposed to full-scale blogging, is low friction and low stakes. Imposing a norm that you can’t just use any ol’ url, looking down on people with insufficently cool urls (as in “Cool URIs don’t change” https://www.w3.org/Provider/Style/URI), puts up too much of a barrier to entry.