There are many scenarios I could posit, but I'll pick one. Let's say your pre-revenue start-up needs a product for sale within 4 months, because that's your company's runway. There's a lot of work to do and limited resources, but if you don't meet that deadline your company has no product to sell and you are out of a job. Are you really arguing that it's poor leadership to say: "we have a hard deadline- the product must be ready in 4 months"?
There are many such hard deadlines in life, and failing to recognize them and motivate your team to meet them is poor leadership.
It's just a date. Ship something by this date. I would suggest shipping more than once before this date. I would not call that a deadline in the normal sense that it's used. Of course, dates should be used as part of the planning process, when necessary. The problem is that most deadlines are overly restrictive and used unnecessarily only to reduce the anxiety of the manager at the expense of the team's motivation. So the net result is less, delivered later.
There are many such hard deadlines in life, and failing to recognize them and motivate your team to meet them is poor leadership.