Each PHP release quietly trims boilerplate, and 8.4 is no exception. Property hooks and asymmetric visibility let you drop a pile of trivial getters and setters without giving up encapsulation.
Performance keeps creeping upward too. The JIT and opcache improvements rarely show up in microbenchmarks, but they add up across a busy request path where every millisecond is multiplied by traffic.
Upgrading is mostly about reading the deprecation list and fixing it before it becomes a runtime error. Treat deprecations as failures in CI and the actual version bump becomes a non-event.