Randolph West shows us how to include computed columns in temporal tables:
As I say in my Back to the future with Temporal Tables session, there are several limitations we should be aware of with this feature. One I had not previously considered is that computed columns are not permitted in a history table. This is a problem if we are converting an existing auditing process to make use of temporal tables, and we have computed columns in our table.
Computed columns are columns that are created from an expression based on one or more existing columns in a table. They are useful if we need to index a portion of a wider column, for example.
Hat tip to Erik Darling for coming up with the solution.