Documentation for .decode() differences between 3.14 & 3.15

Is there documentation anywhere detailing the differences in decode() and how it has changed between 3.14 and 3.15? I have some code which deals with html Post “multipart/form-data” and as far as I recall worked fine in 3.14. However with 3.15.0rc2 this now causes utf-8 can't decode byte 0xff in pos 262. Could it perhaps be that 3.14 used to automatically detect the encoding but 3.15 now doesn’t do that but defaults to utf-8 instead?