It's been a decade since all major Linux distributions adopted systemd as their init system. See https://en.wikipedia.org/wiki/Systemd#Adoption for some information about the adoption.
Spring Boot still supports SysVinit, which is at this point quite archaic, and this support comes with some complexity as evident by the amount of documentation that describes it - including some notable shortcomings of prepending script to a JAR.
Additionally, launch script integration tests are quite complex and resource intensive and take a lot of time when building the project while objectively providing little value considering that shouldn't be the way to run Spring Boot applications in 2025. See this breakdown of build tasks as an example.
This is a formal request to deprecate, and eventually remove, support for SysVinit. There has been some previous mention of this in https://github.com/spring-projects/spring-boot/issues/28453#issuecomment-1325171018.