Free speech does have certain restrictions, which you can easily read about here: https://en.wikipedia.org/wiki/United_States_free_speech_exce...
Contrast with the Basic Law of Germany article 5, that has already come up in this discussion, which outlines specific exceptions in paragraph 2. (For another example, see article 8.)
Real exceptions exist, regardless of whether they are written in the original amendment text or in rulings afterward.
Amendments were added to the body of laws after the constitution, and congress and the supreme court have since added more to the body of laws after the amendments. The cumulative result is what governs citizens today.
Those laws are in fact very specific about their exceptions, as shown very clearly in the wikipedia link. What exactly do you find fuzzy or confusing about them?
Case law, as you linked, is not. It also keeps changing without the underlying documents changing. I don't know how you can look at all those dates marking times high courts had to get involved and still say it's clear.
* https://www.archives.gov/founding-docs/bill-of-rights-transc...
Enjoy.