Can we create custom index on Formula fields (What formula fields are eligible for custom Index)

Custom indexes can be created on deterministic formula fields. Because some values vary over time or change when a transaction updates a related entity, the platform cannot index non-deterministic formulas.

Here are examples of things that make formula fields non-deterministic.

Non-deterministic formula fields can:

• Reference other entities (like fields accessible through lookup fields)
• Include other formula fields that span over other entities
• Use dynamic date and time functions (for example, TODAY and NOW)

These formula fields are also considered non-deterministic

• Owner, autonumber, divisions, or audit fields (except for CreatedDate and CreatedByID fields
1. References to fields that Lightning Platform cannot index
2. Multi-select picklists
3. Currency fields in a multicurrency organization
4. Long text area fields
5. Binary fields (blob, file, or encrypted text)

• Standard fields with special functionalities
1. Opportunity: Amount, TotalOpportunityQuantity, ExpectedRevenue, IsClosed, IsWon
2. Case: ClosedDate, IsClosed
3. Product: ProductFamily, IsActive, IsArchived
4. Solution: Status
5. Lead: Status
6. Activity: Subject, TaskStatus, TaskPriority
• 
  
• Note: If the formula is modified after the index is created, the index is disabled. To re-enable an index, contact Salesforce Customer Support.