Is this mandatory?
In Scrum terms, no, this is not mandatory. There is nothing in the Scrum guide that requires stakeholders to state their availability.
However, there is definitely value in explaining to the stakeholders what their role will be and getting a commitment from them to support the Scrum team.
How can I estimate a client’s availability?
I would suggest the first thing to do is to sit with the stakeholders and explain how Scrum works. Take them through the role of the Product Owner, the sprint review, etc.
Make sure they understand that the effectiveness of the Scrum team will depend to some extent on the contribution that they are willing to provide. The more they can help, the better the product development will likely be.
One approach that I have found helps is for the Scrum Master to pre-book the sprint review meetings many months in the future. This helps the stakeholders to ensure they are available. Also, discuss with them if particular days of the week or times of the day are better for them.