Should SQL Server statistics be included in versioning?
If I am manually creating statistics then yes I would source control those, like I would any other database object. That way if I roll out a fresh environment from source I know it has everything required to function.

Why would I create manual statistics? Well I’d do this if I need control over the maintenance of them, for example if I am using table partitioning then I will manually create statistics and set them to incremental build and with no recompute. This way I can update the statistics manually as part of my ETL, just after I have switched in a new partition of data.