Most has been said but some of my observations (even if redundant):

I remember some file system hierarchy 'wars' but in this case, I must confess that older is really not better.

On my systems, only users within the same group can ever message to my terminal, the last triplet of ls -l /dev/myterm is always ---

cat silently ignores the -u flag because it is already unbuffered. I can not reproduce the buffered behaviour. Nor can I think of a sensible use-case so it's no great loss, really.

I think this was the first time I ever made files with ed (the poems, in particular) and then went back to correct typos with sed. Great fun for bragging rights but I can see why people wrote newer editors.

Apparently only hard links existed at the time. I have never really been able to understand the reason for having two types of links but at least the text gave me one clue: hard links can't be made across devices/mount points (soft links can). Is this really the only reason?

We 'learn' to type ctrl-d when we're on a new line and the buffer is still empty. Used that way, it behaves like an EOF (or whatever) char would do and we get confused. Had never realised you can type ctrl-d half-way a line and then continue typing the same line (in the cat example). I can see this being useful in google meet and etc., or even on irc (push half a line with ctrl-d to signal you're typing and when you finally press enter the whole line is output)