What’s the equivalent mechanism whereby abusive HTTP traffic gets identified and causes an ISP to pay attention?
Is there some sort of greybeard grapevine that old school webmasters can use to get the attention of class A IP network operators?
webmasters as in single operators of httpd somewhere, no, not likely
other network operators who are recognized ASNs and go to NANOG or RIPE or APNIC conferences and meet people face to face, yes.
if you wilfully disregard valid human-written abuse complaints coming from other equally-clueful network engineering persons at other ISPs, you and your ISP will acquire a tainted reputation.
there's various industry back-channels that exist between ISPs to communicate with each other about this sort of thing, I won't go into many details, primarily because there's a high degree of interest of the parties on all sides in keeping the email inboxes and phone numbers uncluttered with clueless single-residential-last-mile-user complaints about their wifi not working right.
"Hey Bob. Bunch of clients on your network keep initiating https connections with servers on my network"
"Gee Bob, sorry about that I'll go see what's causing that right away..."
If folks are escalating to the neteng crews it usually means something is broken via normal channels and it could use attention.
That's social network is exercised for a variety of things:
Abuse, ddos, IP spoofing, botnet C2s being hosted somewhere, malware hosting, complex customer investigations, route leaks, route hijacks, routing loops, congestion, executives home internet issues and more.