As an initial disclaimer, I am a professor, not a lawyer, so my position here certainly should not be relied on as legal advice. FERPA is a law in the United States with potentially harsh penalties (generally falling on the University) for violations. I have formed these opinions reading articles on the Department of Education website, directives from my university, and questions on this stack.
There is little or nothing I could write in an undergraduate or masters-level letter of recommendation absent a FERPA waiver. All of my interactions with these students are somehow related to their academic record. I could confirm directory information about a student, but as a faculty member, I am not the responsible party for confirming that information. Further, since I only teach a couple of undergraduate courses, even confirming I know a student would likely reveal protected information on their academic record.
The case is slightly different for PhD students who are entering the job market. Lack of a FERPA waiver would certainly restrict what I write. However, these letters usually contain objective evaluations of the student's research, which in my field will be at least public working papers by the time I write the letter. I feel that I could discuss my impressions of these papers without disclosing details of that student's academic record. I may (I am not sure) also be able to comment on details of their employment for me as a research assistant or teaching assistant. Again, these points are moot in my case because I require a waiver as a prerequisite to providing recommendations.
I will add that I am likely an exception in that I require any student I write a recommendation for provide a completed FERPA waiver prior to my writing the letter. I further include a notice at the end of each letter that I have provided the information in the letter under a waiver of FERPA rights. Anyone writing a letter for someone they knew as a student in a US university should be doing these things, but I am under the impression many don't.