Other mail client like Outlook has a feature under Calendar i.e - Show calendar as busy, free, tentative, and out of office.
One can set permission for others to view the Free/Busy information in the calendar. Therefore, other users can pick up free time for group schedules, such as meeting. Apart from the permission of viewing Free/Busy information, you need to mark your appointments as busy, free, or others.
You can follow the below steps to find out this feature in Outlook:-
In Outlook if you click the New Items > Appointment on the Home tab;
then just click the File > New > Appointment.
In the Appointment window, click the Show As: box in the Options group on the Appointment tab.
There are five options in the drop-down list of Show As box: Free, Tentative, Busy, Out of Office, and Working Elsewhere. You can select one of them based on your needs.
Please refer to the below article which has detailed information about this feature:-
https://www.extendoffice.com/documents/outlook/1528-outlook-show-calendar-busy-free-out-of-office.html
Not sure of the benefit of this over adding your own entry that says "Working Elsewhere". You also have the option to "Mark Available" or not.
My Customer ask for this, it is very important
Another idea would be to provide an extra option "Mark available for online meetings" as an alternative to the existing option "Mark available". I believe both options together would cover most relevant scenarios when it comes to deciding whether a person can be invited for a meeting.