Also begs the question why SSDs slow down so dramatically and what causes that.
See post #3
@krs, I didn't address that because the poster did not cite any reference for what had been "heard" somewhere. I've never heard about SSDs slowing down dramatically. They work, work well, until they fail, usually catastrophically. Backups are more important for SSDs because they give no warning of pending failure.
The idea on leaving space on drives has a couple of components: 1) The drives themselves want empty space to avoid fragmentation. That factor is much more critical on spinners than SSDs, as I said. The rotational latency and head traverse times make a badly fragmented drive really slow. 2) The operating system needs some space on the drive for scratch space, cache and swap space. Again, on a spinner, you want to avoid fragmentation on those three because of the rotational/traverse latencies. On an SSD, again, it's not so critical, but what you want to avoid is needing space for swap and not having it. So how much to leave depends on how much memory you have and what you are doing. I've read that for the typical user about 5% is plenty, but I suspect a larger drive could be less, maybe a little as 3% on a 2TB SSD. Right now, on my MBP, the swap is shown as 277.8 MB. That probably came from yesterday, when I launched Parallels and Win10 to look at a question someone asked about it.
What will happen if you keep an SSD really full, is that the number of writes to the drive will go up quickly, and with not much room for wear-levelling, the drive will wear out faster than it would if less full. I don't have a practical number for what is optimum, I'm not sure the research is really strong enough to make an absolute call on that. I haven't seen Randy's comments so I won't address them. It could be he has the research.
Personally, no matter the numbers of what CAN be done, I leave a lot of room on all my drives, spinners and SSDs. I have 11 external drives, plus the internal on the Mac, none of which are much over 2/3 full. Rather than pack a drive, I'll go get another one.