In PYTHON-1619 we updated the ObjectId implementation to follow the updated ObjectId spec. As required by the spec we removed the machine bytes and PID portion of the underlying ObjectID and replaced them with a 5-byte random number unique to a machine and process. We should call this out in the changelog for 3.8.0 as it's a potentially breaking change. Applications may have relied on the machine bytes/PID to determine what machine or process created an ObjectID.

The changelog for 3.8.0 says:

ObjectId now implements the ObjectID specification version 0.2.

We should say something like:

Unavoidable breaking changes:

• In order to follow the ObjectID Spec version 0.2, an ObjectId's 3-byte machine identifier and 2-byte process id have been replaced with a single 5-byte random value that is unique to a machine and process.

Reported in https://jira.mongodb.org/browse/PYTHON-1843?focusedCommentId=2276932&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-2276932.