Does the @raise_snmp_trap do anything in @sp_add_alert?
msdb.dbo.sp_add_alert calls msdb.dbo.sp_add_alert_internal, which in turn writes values into the msdb.dbo.sysalerts table.

The sysalerts table insert looks like this:

INSERT INTO msdb.dbo.sysalerts
         (name,
          event_source,
          event_category_id,
          event_id,
          message_id,
          severity,
          enabled,
          delay_between_responses,
          last_occurrence_date,
          last_occurrence_time,
          last_response_date,
          last_response_time,
          notification_message,
          include_event_description,
          database_name,
          event_description_keyword,
          occurrence_count,
          count_reset_date,
          count_reset_time,
          job_id,
          has_notification,
          flags,
          performance_condition,
          category_id)
  VALUES (@name,
          @event_source,
          @event_category_id,
          @event_id,
          @message_id,
          @severity,
          @enabled,
          @delay_between_responses,
          @last_occurrence_date,
          @last_occurrence_time,
          @last_notification_date,
          @last_notification_time,
          @notification_message,
          @include_event_description_in,
          @database_name,
          @event_description_keyword,
          @occurrence_count,
          @count_reset_date,
          @count_reset_time,
          ISNULL(@job_id, CONVERT(UNIQUEIDENTIFIER, 0x00)),
          @has_notification,
          @raise_snmp_trap,
          @performance_condition,
          @category_id)


The @raise_snmp_trap variable is populated and inserted into the table in the flags column.  The documentation for dbo.sysalerts, at Microsoft Docs, says the flags column is "Reserved."  

Since there appears to be no way to access SNMP properties, such as the "community name", "trap destination", etc through the user interface, I would guess SNMP is no longer supported.

In fact, according to Sean Gallardy - Microsoft (Premier Field Engineer):


  ...the definite answer is support was removed in 2000. Yes, at one point it existed but it no longer exists and even support when it existed was dubious at best. All extended stored procedures and associated DLLs were removed.