Using Outlook for iOS for personal email and calendar management
I had a problem with this feature as well and I connected with a Microsoft person who explained that Send Availability still exists, but it has moved. From the mobile Outlook app, you start a reply to a message and instead of clicking on the Calendar Icon (just above the keyboard) you click on the icon of the Person with a check mark, and you're taken to a new page with the header Send Availability, and from there, you click on the dates and times that work for you, then click the Check Mark in the upper right corner. Now you've created the email that you can customize and send to your recipient(s).
Hope this helps.