We have a similar situation - we had a programmer working in a very silo'd manner, writing lots of Python2 code back in 2013/2014. He's since left the company, and while I'm relatively comfortable with Python, there is now a lot of residual Python2 code that will have to be updated to Python3 or rewritten in another language before our next refresh.

By contrast, some of the Perl6 code we have hanging around for various tasks has aged much better.

Python2 in 2015 is/was defensible. There were still a lot of libraries that did not have support for python3 at that time. The inflection point was probably around 2016.