Does setting Query Optimizer Fixes to ON enable Scalar UDF Inlining on a database with compatibility level 140?
No, 'query optimizer fixes' only affects optimizer bug fixes that may result in unexpected query plan changes. It does not enable new features.

The only way to enable scalar function inlining is to set the database compatibility level to 150:


  Enabling Scalar UDF Inlining
  
  You can make workloads automatically eligible for Scalar UDF Inlining by enabling compatibility level 150 for the database. You can set this using Transact-SQL. For example:

ALTER DATABASE [WideWorldImportersDW] SET COMPATIBILITY_LEVEL = 150;

  
  Apart from this, there are no other changes required to be made to UDFs or queries to take advantage of this feature.