/usr/share/irssi/help/lastlog is in irssi 0.8.17-1+deb8u5.
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 | %9Syntax:%9
LASTLOG %|[-] [-file <filename>] [-window <ref#|name>] [-new | -away] [-<level> -<level...>] [-clear] [-count] [-case] [-regexp | -word] [-before [<#>]] [-after [<#>]] [-<# before+after>] [<pattern>] [<count> [<start>]]
%9Parameters:%9
    -:          Doesn't print the 'Lastlog:' and 'End of Lastlog' messages.
    -file:      Output the lastlog to a file instead of the active window.
    -window:    Specifies the window to check.
    -new:       Only displays results since the previous lastlog.
    -away:      Only displays results since you previous away status.
    -level:     Specifies the levels to check.
    -clear:     Removes the previous results from the active window.
    -count:     Displays how many lines match.
    -case:      Performs a case-sensitive matching.
    -regexp:    The given text pattern is a regular expression.
    -word:      The text must match full words.
    -force:     Forces to display the lastlog, even if it exceeds 1000 lines.
    -after:     Only displays results after the given line number.
    -before:    Only displays results before the given line number.
    The pattern to search for and the maximum of lines to display; if no
    parameter is given, the entire window buffer will be displayed.
%9Description:%9
    Searches the active window for a pattern and displays the result.
%9Examples:%9
    /LASTLOG holiday
    /LASTLOG 'is on vacation'
    /LASTLOG -file -force ~/mike.log 'mike'
%9See also:%9 HILIGHT, SCROLLBACK
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