"Exim introduced selectable header positions in Version 4.43.
One option allows you to add your header after all Received:
headers. For optimal SPF draft/RFC2822 compliance, exiscan-acl
adds one other option, called "spf-received". It adds your
header before the first header which is NOT Received: or
Resent-*:, seen from the top of the header stack."
:spf_received: seems to have gone in 4.50, but there is
:at_start_rfc:, which seems to add "immediately before any line that
is not a Received: or Resent-something: header."
This looks even correcter to me (skipping Resent-* headers as well).