It's needlessly complex and O(n), so it doesn't scale well to a
high number of clients nor is it easy-to-scale with the data
structures available to us in pure Perl.
In any case, I see no evidence of either -imapd nor -nntpd
experiencing high connection loads on public-facing sites.
-httpd has never had its own timer-based expiration, either.
Fwiw, public-inbox.org itself has been running a public-facing
HTTP/HTTPS server with no userspace idle client expiration for
the past 8 years or with no ill effect. Clients can come and go
as they wish, and SO_KEEPALIVE takes care of truly broken
connections if they're gone for ~2 hours.
Internet connections drop all time, so it should be harmless to
drop connections w/o warning since both NNTP and IMAP protocols
have well-defined semantics for determining if a message was
truncated (as does HTTP/1.1+).