tsf_user inlineable

As also discussed in the following link, since SQL Server 2019 inlineable scalar functions are an option. This can make sure a query can run parallel. The tsf_user function is not inlineable at this moment. When using this function in a heavy query, it will not be executed parallel as you can see in the images below.

Not inlineable 

When removing the use of variables in the function, the function is inlineable and the queries will run parallel when needed:

The script I have used is:

SET QUOTED_IDENTIFIER ON;
SET ANSI_NULLS ON;
GO
CREATE OR ALTER FUNCTION tsf_user
()
RETURNS tsf_user
AS
BEGIN


   --control_proc_id:      sql_tsf_user
   --template_id:          sql_tsf_user
   --prog_object_item_id:  sql_tsf_user
   --template_description: Returns context_info or system_user


   RETURN CASE WHEN CONTEXT_INFO() IS NOT NULL THEN
                  CASE WHEN LEN(REPLACE(
                                          CONVERT(
                                                    VARCHAR(128),
                                                    CONTEXT_INFO()
                                                 )COLLATE Latin1_General_BIN,
                                          CHAR(0),
                                          ''
                                       )
                               ) <> 36 THEN
                          REPLACE(
                                    CONVERT(VARCHAR(128), CONTEXT_INFO())COLLATE Latin1_General_BIN,
                                    CHAR(0),
                                    ''
                                 )
                       WHEN SERVERPROPERTY('EngineEdition') <> 5 THEN
                          REPLACE(
                                    CONVERT(VARCHAR(128), CONTEXT_INFO())COLLATE Latin1_General_BIN,
                                    CHAR(0),
                                    ''
                                 )
                       WHEN REPLACE(
                                      CONVERT(VARCHAR(128), CONTEXT_INFO())COLLATE Latin1_General_BIN,
                                      CHAR(0),
                                      ''
                                   )NOT LIKE '________-____-4___-____-____________' THEN
                          REPLACE(
                                    CONVERT(VARCHAR(128), CONTEXT_INFO())COLLATE Latin1_General_BIN,
                                    CHAR(0),
                                    ''
                                 )
                  END
               ELSE SYSTEM_USER
          END;


END;
GO

Perhaps this is something you can use in a new version.