Which is only useful in the narrow case in which your target supports IPv6 and doesn't treat each /64 block as one address for the purposes of rate limiting. This combination seems quite rare.

Why would you think it's rare? I've seen places running multiple projects were developers were barely aware that the sysadmins gave everyone an IPv6 address, and therefore the rate limiting has to be IPv6-aware.

It's trivial to fix once it becomes a problem (at least in our case it was), but I wouldn't expect it to be an uncommon mistake.

It’s pretty easy to acquire blocks larger than /64.

if you rate-limit, you should probably be rate-limiting on the /64 prefix and those are about as hard to get as individual v4 addresses.

It would be trivial to rate-limit the whole subnet then.

Maybe, but that does not mean a lot of ISPs haven't done that.