Checking the code reveals that :

-- validate @publication_type
    --
    if (@publisher_db is NULL or @publication is NULL) 
        and @publication_type is NULL 
    begin
        raiserror(20587, 16, -1, '@publication_type', 'sp_replmonitorhelpsubscription')
        return (1)
    end

This means that if @publication_type is NULL then it will throw an error which is expected.

Even on 2014, it is throwing same error.

Checking the code reveals that :

-- validate @publication_type
    --
    if (@publisher_db is NULL or @publication is NULL) 
        and @publication_type is NULL 
    begin
        raiserror(20587, 16, -1, '@publication_type', 'sp_replmonitorhelpsubscription')
        return (1)
    end

This means that if @publication_type is NULL then it will throw an error which is expected.

Even on 2014, it is throwing same error.

Also, you just need to run with

sp_replmonitorhelpsubscription @publication_type = 0 -- for Transactional publication. since the data type is INT.