TL;DR: matrix.org and vector.im receive a lot of private, personal and identifiable data on a regular basis, or metadata that can be used to precisely identify and/or track users/server, their social graph, usage pattern and potential location. This is possible both by the default configuration values in synapse/Riot that do not promote privacy, and by specific choices made by their developers to not disclose, inform users or resolve in a timely manner several known behaviours of the software.
https://gist.github.com/maxidorius/5736fd09c9194b7a6dc03b6b8d7220d0
https://archive.fo/sgnPn