They have a responsibility to let you know, yes. And they have to actually let you take it. Beyond that should be personal responsibility.

Here are my expectations and how I normally experience pto policy at work.

• HR has in writing company PTO policy. When and how to apply, how many per year, rollover policy.
• HR provides friendly reminders in email to use it or lose it and the deadline is coming up.
• HR provides a decent software system that tracks your PTO balance and history and is easy enough to use to request time off
• HR reminds managers to approve PTO unless there is some issue in which case HR should help handle

If a company does the above then the employee has no one to blame if their days are lost.