They wrote it, so they could close its source regardless of the license (which was actually Apache 2 and APSL before that).

It looks like 10.9 (released in 2016) was the last release on https://opensource.apple.com/ that included launchd. The original author left Apple in 2014, so maybe it had something to do with that (similar to why they abandoned MacRuby and CUPS), though one would think that they still update it internally.

They do. Launchd/Mach/xpc are still critical technology pieces that underlies every service and nearly every single framework (since most of them just delegate to a service).

It’s gotten smarter over the years. Launchd now supports complex Jetsam configuration parameters which let you specify RAM and CPU thresholds when your daemon is active and not (to be robust against memory leaks)

Apple owns launchd, though, and it was available under APSL and Apache before going closed-source.