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.
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"
You can voice your displeasure over this change in this Outlook Uservoice thread: Bring back Advanced Find to Outlook Search Tools.