It's worth noting that as far as I can tell, Linux didn't support CAP_NET_BIND_SERVICE until 2.6.24[1], which was released in January 2008[2].
Exim itself dates from 1995[3].
I'm not really up to date on the use of capabilities, but it would seem that it can be setup before running the main processes anyway[4] using the setcap command (not sure how portable this is on other platforms, eg. BSD's) and it would appear to be a distribution/packaging issue in that context anyway.
There is also always the possibility of setting the port used for SMTP connections to a port higher than 1024 anyway, and using iptables/firewalld etc. to forward port 25 to that unprivileged port, as also discussed in [4].
Of course, neither of these options help in the specific case of needing to access user's home directories, either to read .forward files or deliver mail there directly.
