That isn't a justification; if the students think that it helps they can take notes of their own initiative; handing out a copy of the slides doesn't disadvantage anyone. The lecturers are just making a petty power play because they want to feel important standing in front of a bunch of eager young students

There simply isn't any pedagogical justification for withholding information post-lecture that was deemed important enough to be included in a lecture. Everyone knows this material is critical to understanding the course. If professors want simple optimisations to help students learn then they would be doing something like organising lecture material to promote spaced repetition.

Does it? I once had a course where for some reason administration cut the course hours in half, without cutting the program. Teacher apologized in advance and asked us to take notes and read them at home later. After every lecture, I kid you not, I was lucky if I could remember the topic that was read to us. Absolutely no facts. It was lectures on topics I was slightly familiar with. And the speed of the lecture wasn't even as high (sic!) as the speed of some youtube lectures I have watched later.

You need to have time to process, but when you have to take notes there is a speed at which you just skip processing and instead all of your focus is in transcribing as efficiently as you can. I imagine, the speed differs from person to person. For the most of this course, the teacher hit this speed. And she knew she would hit it. So it seems universal.

If current student generation is worse at taking notes by hand, it could be a real disadvantage for them

You only have one language processing centre. You are either copying from the screen or listening to the professor, not both.

It also means instead of listening youre busy writing down stuff in time before the next slide

except you don't have time to do that in the moment, since you're busy scribbling down formulas trying to keep up