The "Hide last activity info after the number of days" is dependent on the "Display last activity"->"Hide after days" setting (see attachment).  For correct usability it should therefore be displayed as such, by making the "Display last activity" a radio-button list, and the number of days appear after the "Hide after days" option.  i.e.

-- Display last activity ----
O Always display
O Hide after days: [100]
O Always hide
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This is the sort of thing which is standard, for example on printing (see attachment) where the fields specifying which pages to print are only available when the "Pages" option is selected.

How bad can this be as it is?  Well, I thought the field was broken, because it did not behave as I expected and so raised a ticket!  If usability is so bad that I need to raise a ticket to understand it, then something is definitely wrong!