I’ve been using Arch for about 15 years or so, and yes, I build up cruft… in my home directory ;-). The system itself is remarkably good at keeping tidy. The one spot to keep an eye on is /var/cache/pacman, as that’s where it stores every package you download before installation and it won’t delete it without you asking it to.
Any new config file will be saved with a .pacsave extension, so you’ll want to keep an eye out for those, but that’s basically it