If you are searching for "IsSaaS" and "Business Central" because IsSaaS always returns true on a sandbox, then hopefully you have found this post.
odeunit "Environment Information" contains function IsSaaS. In BC 18, this will return TRUE on any sandbox, whether the sandbox is in cloud or not. How can you have a Sandbox which is not in the cloud? Two ways: In a Docker image, or you can mount a tenant as a Sandbox (Mount-NAVTenant -EnvironmentType Sandbox).
In order to be able to simulate SaaS on a Docker image, the IsSaaS function was changed in BC18 so that it will always return TRUE on a Sandbox.
If you want to check for SaaS in an OnPrem Sandbox, then use the function IsSaaSInfrastructure instead.
If you just read this, it probably will not make much sense, but if you got here via a Bing search then hopefully this is the answer you were looking for :-).