Many file formats record creation timestamps, like in image, document, video, audio, executable, archive, and so on. If you create these formats and send the file to others, they would at least know when you created the file. Sometimes they even include timezone info, so they can even know something about your geo location. This applies to network protocols, application communications, database records and so on. Even Git will record your timestamp and timezone info for every commit, and it's very difficult to completely change or remove these info.<p>You might think it's a trivial thing, but it actually tells a lot about you. If someone can trace your activities through time, it's essentially a detailed profile of you, and they can learn how you live and work. Sometimes it can even be used to de-anonymize you by cross referencing with your "real" online identity.<p>In general it's impractical for users to fully understand what kinds of meta data were included in each file format or send by each application. EXIF data is often included in image files generated by cameras or image editing software. Your full file path to a source code file may be included in the executable you compiled, and it may leak your personal information. Your operating system may send regular health report to its company. A proxy service may append your real IP address in HTTP headers. Even for some encrypted services, they don't encrypt or sign everything. Like 1Password in the past didn't encrypt the URLs of your saved login sites. TLS 1.2 doesn't sign the cipher suites. TLS 1.3 doesn't encrypt client certificate.<p>Most of these software and protocols were not designed with privacy as a primary concern. Even they do, there are info that they decided to be okay to leak. However, it should be up to the users to decide whether the design decisions were reasonable for their own use case. Even many of these meta data leak seem like targeted surveillance, it's actually scalable and can be adapted to mass surveillance.