> The section on multi-column indexes mirrors how I was taught and how I’ve generally handled such indexes in the past. But is it still true for more recent PG versions?

No, it isn't. PostgreSQL 18 added support for index skip scan:

https://youtu.be/RTXeA5svapg?si=_6q3mj1sJL8oLEWC&t=1366

It's actually possible to use a multicolumn index with a query that only has operators on its lower-order columns in earlier versions. But that requires a full index scan, which is usually very inefficient.