I found 2 references here for migrating D8 to D8, but both are very outdated.
Certainly there will be a need to do D7 -> D9 or D8 -> D9 migrations for many, but there are also current use cases. One is migrating from a multisite to individual sites.
Mine is the site restructuring its content types, and it would be far simpler to add custom code to a source plugin than to do all of it in custom code.