When is a not updated software considered deprecated in the academic world?
I don't think there is such a standard, certainly not one applicable to all of academia. For your thesis, I would suggest that it might be more useful to talk about "recently updated" / "currently developed" software, rather than labeling the rest as obsolete.

"Deprecated" signals that the creator no longer recommends using the software. I can't think of any real examples for a full software package (i.e. going beyond deprecating part of an API), but I guess it might happen if the software is known to produce bad results.

Software might be "obsolete" if it has been de facto replaced by a better option, but certainly not just because it's old. Even then, the old option might be good enough. Visit a couple of (established) experimental labs in the sciences, and you'll likely see lots of old equipment still running old software. Changing it all out would just be needless overhead, and not be guaranteed to help. There's almost always a better place to spend the money you might have... From another angle, if you consider numerical algorithm implementations and libraries as tools, there's a number of them that have gone decades without updates, yet are still routinely imported and used in new software development.