[Discussion] GTFS Realtime Versioning

In recent discussions regarding Realtime spec, the community seems to have encountered difficulties in reaching consensus, leading to the topic of versioning being raised in the recent [PR#504](https://github.com/google/transit/pull/504).

To address this, we are opening this issue dedicated to discussing GTFS Realtime versioning.

The following questions may help us begin to think about this significant topic:
- If we implement versioning (i.e., a mechanism for incrementing version numbers), should this mechanism allow for breaking changes for producers by permitting new versions to be incompatible with older versions? For example, should it allow for the complete removal of certain elements or the redefinition (modification) of content from previous versions in the new version spec?
- Compared to the existing mechanism of using the [deprecated = true] tag, what are the advantages and potential risks of implementing versioning?