I do not get (aha) where you saw limit_rate only applies to the GET
But yeah limit_rate applies to resposnes.
Rate limiting only properly applies to sender, in your case the client,
which is the sole entity ablte to properly craft its requests to contain
specified amount of data/time period.
The only thing you can limit on intermediaries/receiver is
connections/packets, because it is network-related structures which are
trivial to handle/buffer.
Rate-limiting on a transmitting/receiving end requires buffering content
(not envelope, so that means application logic/DPI), and re-crafting
forwarded/processed content into suitable network envelopes.
Way too expensive/dangerous/demanding.
You can limit incoming transmissions in nginx based on connections
(limit_conn) or requests (limit_req).
You can limit incoming transmissions at TCP level in firewalls surch as
iptables based on connections and/or packets.
My 2 cents,