Are all the MTA's exams based on the latest OS available?
Depending on which MTA you are taking you will need to specifically look at the objectives listed on Microsoft's page.
They also have some good info here on their FAQ page:
You won't find a specific OS called out. Microsoft even throws out a disclaimer for the Microsoft Press and Microsoft training books (as shown below).
"All learning materials related to exams, including Microsoft training and Microsoft Press resources, are developed independently of exam content, because the exams are intended to assess competence when using the technology, not the effectiveness of training materials or learning. Qualified candidates must have sufficient experience in working with the technology to pass the exam. Training is a great first step in preparing for a Microsoft Certification exam, but it is not required and does not guarantee that you will pass the exam. Experience is required."
In my experience they do incorperate a slight mix of the newer OS's.
I wouldn't worry too much about unsupported or legacy OS's.
Shoot for the current or previous revisions unless you are specifically needing to know them in reference to a particular technology such as migrations or update paths respectively.
You can also note the "Published date" listed on Microsoft's exam details page.
If the publish date is prior to the release of the OS in question the I would focus my efforts on the previous version.
As the exam evolves and gets revisions we will be updating our shows to reflect those changes as well.
Happy Studying. I hope this helps. Let me know if you any further help.
Historically when Microsoft puts out an exam it is the most current operating system or technology. I agree with Cherokee in checking the objectives for requirements, however I will say that they will be requiring Windows 10 as of August 31, 2016 for the 98-349 Windows OS Fundamentals exam:
In this "update" to the exam they have removed some of the features that are specific to Windows 7 like Aero, Gadgets...etc... The update also changes the focus to from "understand" Topic A&B to "configure" Topic A&B. So more stress is on using the technology. As for the other MTA's I would expect them to update as well just pay attention to change documentation like what can be found in the link above for 98-349. I hope this helps.
Thanks Cherokee and Wes!