No, this did not reply a figure handle in R2013b:
The plot command replied a line handle and it still does in R2020.
With the HG1 interface of R2013b the line handle was a double as all GUI handles. With HG2 the handles are specific types.
If this line handle is used as handle to this line only and the code does not assume, that it a double, the existing code should run without changes. So please post the part of the code, which is failing.