Update September 18 2017: As of Insider Fast build 1710 (16.0.8530.1000), Advanced find is back on the Search Tools menu. This may be a temporary reprieve, until the new search tools are ready (which is how it should have been to begin with.) This includes Expanded Find Pane and More fields.
Users with the Insider builds were the first to notice that Advanced Find was missing. This wasn’t a bug; it was intentionally removed in preparation for improvements in search coming later this year. Based on data available to Microsoft (from people who agreed to share usage data with Microsoft), a very small percentage of users used Advanced Find. (Honestly, which is used more, Publisher or Advanced Find?)
On September 15, 2017, Microsoft released an update that removed it from Current Channel users. The outcry was loud enough that Microsoft released an update enabling it on September 18. Yes, they do listen!
While this is true, those who use Advanced Find rely heavily on it and it does one thing Instant Search can’t do: partial word searches. Plus, when Query Builder is enabled, users can easily create AND/OR queries.
In addition to removing Advanced find, Microsoft also removed the Expanded Find Pane and the fields from the More button, as well as Recent Search history.
Instant Search and Search folders can replace some of the functionality of Advanced Find (except for partial word searches). For better results, refine your Instant Search queries using logical operators AND, NOT, OR, <, >, =. For more information, see "Outlook Instant Search Queries"
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You can voice your displeasure over this change in this Outlook Uservoice thread: Bring back Advanced Find to Outlook Search Tools.
How does one change his or her vote?
I don't think you can. You can vote again using a different browser or a private browser. (Yeah, the results are that accurate! :))
Imagine if you bought a car that came with a CD player. Then, one day while driving to work you notice that your CD player is gone; replaced with an auxiliary input jack. You call the dealership and they say "Well, the OEM found that only a very small percentage of users were actually using the CD player, so they changed it. Excuse me? I BOUGHT something with a certain set of features and I EXPECT those features to remain for the entire time I own the product. Just because it's not a physical product, doesn't mean the maker can change it whenever they want. Where does it stop? What if I have a pacemaker with certain features that are saving my life, but those features are "rarely used"? Are they going to give me an over-the-air update to remove those features too? If a company wants to change, modify, or improve their product they are free to do so. I welcome innovation! But changes should only impact future customers, not existing ones, unless those existing customers explicitly opt-in to the changes.