Having the capability to compute a UTC timestamp as the default value for a column is a handy feature of most popular database solutions. Does Google Cloud Spanner support this? If not, is this a possible roadmap item?
Cloud Spanner doesn't actually allow any default value to be specified regardless of type. This means the implicit default is Null, or Error, depending on whether to column was specified with NOT NULL
Cloud Spanner internally stores a 'timestamp' of when a row was committed, but it doesn't expose this directly. It also doesn't behave like a default value (set once), so unfortunately the answer to your question is currently no.
Definitely something for the team to consider.