Making sense of sys.objects, sys.system_objects, and sys.sysobjects?
As noted in my previous post sys.sysobjects is deprecated:


  Note taken from sys.sysobjects (Transact-SQL)
  
  This SQL Server 2000 system table is included as a view for backward compatibility. We recommend that you use the current SQL Server system views instead. To find the equivalent system view or views, see Mapping System Tables to System Views (Transact-SQL). This feature will be removed in a future version of Microsoft SQL Server. Avoid using this feature in new development work, and plan to modify applications that currently use this feature.


You would now have to combine sys.system_objects and sys.objects to retrieve all items that are stored in the deprecated sys.sysobjects system table.

SELECT * FROM sys.system_objects 
UNION ALL
SELECT * FROM sys.objects AS o


Result:

(2171 row(s) affected)



sys.system_objects 


  Contains one row for all schema-scoped system objects that are included with Microsoft SQL Server. All system objects are contained in the schemas named sys or INFORMATION_SCHEMA. 

sys.objects


  Contains a row for each user-defined, schema-scoped object that is created within a database, including natively compiled scalar user-defined function



For example sp_MScleanupmergepublisher is ìs_ms_shipped but not in the sysschema (it is in dbo) so it's in sys.objects and not sys.system_objects. This is possibly because  it is a shell for sys.sp_MScleanupmergepublisher_internal and is probably defined on instance creation. If you have SSMS right-click on the dbo.sp_MScleanupmergepublisher system stored procedure and then select Modify:  

USE [master]
GO
/****** Object:  StoredProcedure [dbo].[sp_MScleanupmergepublisher]    Script Date: 19.12.2017 12:12:13 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER OFF
GO
ALTER procedure [dbo].[sp_MScleanupmergepublisher]
as
    exec sys.sp_MScleanupmergepublisher_internal