What is a" proposed arrival date"?
A planned date? The exact date?

Can it be the date that I plan to arrive but I might change it in the future?

Question

What is a" proposed arrival date"?
A planned date? The exact date?

Can it be the date that I  plan to arrive but I might change it in the future?

Answer

Yes, "proposed" means your plan, but not necessarily certain.
But, when a hotel asks for your "proposed arrival date", they probably take is as your request. If they can accommodate you, they may then ask whether you want to book that date.

Answer

It would be the planned or suggested date. More often it would be the suggested date.