I've dealt with systems using floats as IDs in their DB. Better than when the monetary values were floats

Actually in the CG shader language you can index arrays with floats to get automatic linear interpolation, but I digress...

Twitter used to use JavaScript numbers to represent various ids in their JSON API. These are, per the JavaScript spec, double-precision (64-bit) floating point numbers. You can only really use these to represent 53-bit integers, so they had to add string versions.

(see https://developer.twitter.com/en/docs/basics/twitter-ids)