I have now transferred all findings to the list. See https://pagure.io/fedora-server/issue/96#comment-841351. Many thanks to Kevin, who contributed all the specific configurations of our "big" core services.
As I see it, the result is mixed.
1. As far as we know, either a service has set the timeout value to infinity, or has set no timeout at all. All the latter rely on the system default value.
2. At least most of our "huge core services" as virtualization, PostgreSQL, IPA, etc. which are "suspected" of taking longer for a clean shutdown, have the timeout set to infinity.
3. But some of our "huge core services" did not specify a timeout value, e.g. all the container services (for information: CoreOS, the "pod man container OS", decided to keep the old timeout value).
4. There are a number of "alternative" services as e.g. MariaDB database, which may need longer as well. We have no information yet about their timeout value. These are not "core" (in the sense we tested everything, and it is a potential release blocker), but nevertheless everyone would expect that these work flawlessly and safely on Fedora Server, nevertheless. So we should take these into consideration as well.
5. There are some services that must rely on a system default, as WildFly or various collaboration software, because there is (currently) no rpm
Maybe I'm being overly cautious. But I prefer "better safe than sorry" and like the decision of CoreOS - at least as long as we don't have better data and measurements.