A casualty of acquisitions, I think. Linux Academy got bought by A Cloud Guru, who then got bought by Vista Equity along with Pluralsight. I think the material lives there now [0].

Packt does have "Linux Service Management Made Easy with systemd" as of Feb 2022, but I've not yet read it.

[0] https://www.pluralsight.com/cloud-guru/courses/mastering-sys...

[1] https://www.packtpub.com/product/linux-service-management-ma...