I'm not sure how to interpret the pypi download stats. There is a very
regular weekly rhythm and 80% of the downloads are for Linux which I
don't think is representative of the userbase. It looks to me more
like some servers are using sympy and installing it from pypi
thousands of times per day.
In practical terms, maintaining an LTS just means that if a
substantial problem arises with using SymPy 1.5 on Python 2.7 we could
add a fix for it on the 1.5 branch and release 1.5.x with that fix. I
can't see what that problem would be though apart from, say, an update
to pypi that means 1.5
