To find that post, I have tried to search for "[decimal-expansion] [elementary-number-theory] [divisibility] 9 is:q" math.stackexchange.com/…
Maybe omitting is:q would make sense too. Maybe also simply choosing those three tags and then try to sort by votes or check the frequent tag would work.
It is kind of easy to suggest alternative ways to find some specific posts after it was already found in some other way. :-)