They're separate programs and system services which all more or less just do their own thing, just developed under the systemd umbrella. So it can't be "toggled", you can just not use the parts of systemd you don't want.
But it's meant to work as a cohesive system when everything in systemd is used together.
FWIW, I think it's great that someone is trying to make a coherent set of system services for Linux. Things tend to interoperate better when they're explicitly written to work together than when every component is meant to be hacked to work with arbitrary other services through shell script soup.
Right, are any of the things mentioned in the GP really required if you only want this init?
I know they've attached all these projects to the systemd brand because they thought it would beneficial but it's hard not to wonder if we could avoid all those discussions if the umbrella project was called something different...
It's great that the same someone has formed a company called Amutable which has the sole purpose of converting Linux to a locked-down immutable OS where the users doesn't have the key.
Also it's interesting that a set of simple interfaces have worked for so long. Maybe they did something wrong that it didn't break?
See: https://www.amutable.com