Collecting MSBuild binary logs from MSBuild as described on https://msbuildlog.com may collect your source code, secrets from your source code and environment and store them in the .binlog file, exclusively for the purpose of making .binlog files useful for build analysis. Treat .binlog files like you treat your source code and environment variables. Do not share .binlog files publicly unless you are confident they contain no secrets or private information. See details at https://msbuildlog.com/#security
The MSBuild Structured Log Viewer app opens .binlog files but it makes no web or cloud requests and all viewing and processing is done on the local machine. It will not send any information anywhere.
The only network call the binlog viewer makes is to self-update via https://github.com/KirillOsenkov/MSBuildStructuredLog/releases.