Yes, employee 45 SHOULD show up in this list, because nobody reports to them.

Add another record where someone reports to 45, then 45 will not appear - even if you delete its parent record:

create table #employees (employeeid int, reportsto int)

insert #employees select 10, null

insert #employees select 45, 10

insert #employees select 50, 45

--here is the original query - we find employee 50 as the only one who does not have anyone reporting to them

select *

from #employees

left join #employees as subordinates on #employees.employeeid = subordinates.reportsto

where subordinates.employeeid Is Null

--now delete the reportsto record

delete #employees where employeeid = 10

-- we still get the same answer, 45 still does not appear, even though its parent is deleted

select *

from #employees

left join #employees as subordinates on #employees.employeeid = subordinates.reportsto

where subordinates.employeeid Is Null

drop table #employees