C question

is a byte with all zeros equivalent to the integer value (signed or unsigned) zero, in terms of portability?

Such that, is memset(&i, 0, sizeof(int)); equivalent to i = 0;?

>>29

And C was both a systems and applications language in the 70s or 80s

Yes, but not as early or as important as you think it is. It's not like Java, which came out in 1995 and was a huge thing for web applets in the 90s. C didn't become popular until the Sun workstation came out and became more popular when Microsoft picked C for Windows.
https://en.wikipedia.org/wiki/Sun-1

Where are all of these systems and applications written in ``C without structs'' or ``C without compound declarators''?

The earlier compiler does not know about structures at all: the string "struct" does not appear anywhere. The second tape has a compiler that does implement structures in a way that begins to approach their current meaning. Their declaration syntax seems to use () instead of {}, but . and -> for specifying members of a structure itself and members of a pointed-to structure are both there.

Neither compiler yet handled the general declaration syntax of today or even K&R I, with its compound declarators like the one in int **ipp; . The compilers have not yet evolved the notion of compounding of type constructors ("array of pointers to functions", for example). These would appear, though, by 5th or 6th edition Unix (say 1975), as described (in Postscript) in the C manual a couple of years after these versions.

C was an unfinished toy project in the 70s.
https://www.bell-labs.com/usr/dmr/www/chist.html

it was in fact TOO HIGH LEVEL to be used on 8-bit microcomputers.

It was too bloated, not too high level. BASIC is higher level, but it's smaller (and usually interpreted).