/usr/share/zsh/functions/Zle/incarg is in zsh-common 5.3.1-4.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 | # Shell function to increment an integer either under the cursor or just
# to the left of it.  Use
#   autoload -Uz incarg
#   zle -N incarg
#   bindkey "..." incarg
# to define it.  For example,
#   echo 41
#        ^^^ cursor anywhere here
# with incarg gives
#   echo 42
# with the cursor in the same place.
#
# A numeric argument gives a number other than 1 to add (may be negative).
# If you're going to do it a lot with one particular number, you can set
# the parameter incarg to that number (a numeric argument still takes
# precedence).
emulate -L zsh
setopt extendedglob
local rrest lrest num
rrest=${RBUFFER##[0-9]#}
if [[ $RBUFFER = [0-9]* ]]; then
  if [[ -z $rrest ]]; then
    num=$RBUFFER
  else
    num=${RBUFFER[1,-$#rrest-1]}
  fi
fi
lrest=${LBUFFER%%[0-9]#}
if [[ $LBUFFER = *[0-9] ]]; then
  if [[ -z $lrest ]]; then
    num="$LBUFFER$num"
  else
    num="${LBUFFER[$#lrest+1,-1]}$num"
  fi
fi
[[ -n $num ]] && (( num += ${NUMERIC:-${incarg:-1}} ))
BUFFER="$lrest$num$rrest"
 |