What is the chatLengthEstimator.cmsketch file located in the Library/Suggestions folder? (Mac OSX)
The files in ~/Library/Suggestions are used internally to support some of macOS's machine learning features, like Spotlight Suggestions, Suggested Contacts, and data detectors. These files are not being suggested to you.

A .cmSketch file probably contains a count-min sketch data structure. It does not contain any human-readable data. Given the name of this file, it is probably used to estimate how often, or for how long, you chat with your contacts on iMessage.