Looks like it’s fixed. Worked for me: https://ftp.arl.army.mil/~mike/ping.html

No, the link he provided in the document to the original PING source. It's supposed to go to http://ftp.arl.army.mil/pub/ping.shar But ends up at https://www.arl.army.mil/www/default.cfm

Looks like it's viewable here: https://github.com/iputils/iputils/blob/master/ping.c

obviously not the original source code from 1983.

nope, but derived.

this is pretty close:

https://www.tuhs.org/cgi-bin/utree.pl?file=4.3BSD/usr/src/et...

    pr_pack( buf, cc, from )
    char *buf;
    int cc;
    struct sockaddr_in *from;
    {

I know this ship has sailed, but after reading through the code you linked to, I finally understand the merit to this old-style C function definition syntax, and actually think it might look nicer than the modern way of doing the same thing for longer parameter lists:

    pr_pack( char *buf,
             int cc,
             struct sockaddr_in *from )
    {

It reminds me of how in TypeScript, if a function gets too many arguments, I'll split them out into an interface, and use destructuring to get to each element in the actual parameter list for convenience.