> Cool URIs Don't Change
What makes a cool URI? A cool URI is one which does not change. What sorts of URI change? URIs don't change: people change them.
Tim Berners-Lee, Hypertext Style: Cool URIs don't change, 1998. Leave the file extension out, leave the CGI directory out, leave the author's name out, leave anything out that a future reorganization might invalidate. A good value to have.
This site's slugs follow this rule. notes.11tydata.js derives every URL from the file's slug alone, so a note keeps its address for as long as the filename does, independent of category, theme, or any future retagging.