I searched 6M LoC of Python code at Dropbox and found only three uses. They seem legit. Two are about formatting a number that's given as a float, deciding whether to print a float as 42 or 3.14. The third is attempting a conversion from float to integer where a non-integer must raise a specific exception (the same function also supports a string as long as it can be parsed as an int).

I don't doubt we would get by if is_integer() was deprecated.