Some years ago I read a statement in the Apple OS X HIG, that software installers should avoid reboots if possible because they break the system's availability. However, I can't find the source anymore. Could please someone point me there? Is there a similar rule from Microsoft?
I don't know about macOS but the Human Interface Guidelines for iOS certainly do include a recommendation to avoid reboots as part of the onboarding process:
Don’t encourage rebooting. Restarting takes time and makes your app seem unreliable and hard to use. If your app has memory or other issues that make it difficult to run unless the system has just booted, you need to address those issues.
Microsoft provides similar guidance for setting-up Windows applications:
Avoid restarting Windows. Most programs should install without restarting Windows. The primary reason program installations or updates require a system restart is that some of the files involved are currently being used by a running program. In this case, a better alternative is to make users aware of the situation, allow users to close these programs, and retry the action.
There's a similar suggestion (rule) within the iOS HIG and educational materials to developers, from Apple. It's express meaning is slightly different from what you're getting at.
Apps that recommend that users restart their iOS device prior to installation or launch may be rejected
from this page
The overtone I suggest is being expressed... it's not necessary, in an ideal world, and Apple's ecosystem and products are the ideal world.