Below are the few possibilities that a field can turn into indexing.
Possibility 2: One way that custom indexes get created is by Salesforce’s auto-indexer. The platform constantly analyzes custom index candidates, and when it sees that a custom index would make a noticeable difference in the response time for one or more queries, it automatically creates the index. 
Possibility 1: As
per the cheat sheet, the below fields are auto indexed by platform
Possibility 3: By raising a case with SF support.
NOTE: Don't presume that all lookups are auto-indexed, see the below screenshot for reference
Resources :
2. Private.Blog
tHiNk gooD and dO thE bEsT.........MANJU NATH 🌝
Thank you for posting this valuable information. Looking forward for more posts.
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