Please do continue to make suggestions! Propshaft is an internal detail that the Rails team is proud of, but it it is essentially invisible to Rails developers. If you look closely at the HTML that is generated by a typical Rails app, you will see things like
<link rel="modulepreload" href="/assets/application-bfcdf840.js">
Propshaft is the component that creates that hex number before the dot. Before Propshaft, this was done by Sprockets, which had the ability to invoke preprocessors like coffee-script that no longer are popular, So the big changes is that you can no longer do something you weren’t likely to want to do anyway; and the Rails code base got smaller/simpler.