No UI version, but you can retrieve it from the (undocumented) Observable API.
For example:
https://api.observablehq.com/document/@mbostock/five-minute-introduction
has the version number 344.
You could also get a specific version by appending @versionnum to the end. So if we wanted version 343 of the example above:
https://api.observablehq.com/document/@mbostock/five-minute-introduction@343
However, since it’s undocumented, I’m not sure how long this will be supported for, and there’s no way to see all the version numbers you have permission to access (e.g. @100 for the notebook above will give a 404).