fraid I generated a tl;dr for this rather verbose article:
“Home directories are a mess because too many apps ignore XDG spec and dump dotfiles everywhere. The problem isn’t just legacy software—new apps do it too, often out of ignorance or laziness. Windows has similar issues with profile folders. Fixing it requires devs to actually follow standards, but many resist due to inertia or ‘my way is better’ thinking. Users should push back and demand proper XDG compliance to keep $HOME clean.”
In case you don’t already know about it, paccache (part of the pacman-contrib package) will let you easily remove old packages from the pacman cache