I, too, have a changelog and archives on all lists and every few years look for inactive lists. Note that the ".list" file and changelog will change with just an unsubscribe ... and an unsubscribe should not be considered activity, IMHO.   However, the archives is also not definitive wrt inactivity.  For example, I have a number of lists which are not used for posts, but are back-office sublists.

I emphasize that the lists for my site belong to their sponsor and a sponsor is always an owner (at my site).

So once I've recognized a probable inactive list, it's my task to ask the sponsor, not to decide if this is truly a list that may be discarded.  This also gives the sponsor/owner a chance to get a copy of current subscribers and archives, should they want them.    I also don't discard lists but have a script that squirrels away interesting files/folders.   As I have used this to restore only once in the past 15 years of using it, I'm not recommending (or providing) it! ;-)

Trying to be nice to the sponsor/owner, I look to see if they own other lists that might be inactive.   On my Linux server this script finds all lists owned by someone:

#!/bin/bash
#
# Name: scanowners.bash
#
# Purpose: Scan all LISTSERV lists for a target
#          (target looked for on "owners= lines only)
#
# 2015-03-04 [log in to unmask]
#

home="/home/listserv/home"
bin="/home/listserv"

let "count=0"
let "nlists=0"

for i in $home/*.list; do
   let "nlists+=1"
   $bin/listview -h $i               \
     | grep -i "Owner= "             \
     | grep -i "$1"                  \
     > /dev/null && echo $i && let "count+=1"
   done
echo "$1 found on" $count "of" $nlists "lists."

Hope this helps!   (lots of manual work, so it's really a low priority task)