A recent update changed the way prefix option works. Generally, you can replace multiple prefixes (for example if you had same "prefix:" under retention and consistency) with archive_name_format under storage.
Example:
[old way]
storage:
archive_name_format: 'homepage-{now}'
retention:
prefix: 'homepage-'
consistency:
prefix: 'homepage-'
Can be replaced with:
[new way]
storage:
archive_name_format: 'homepage-{now}'
If you need different 'prefix' values for some reason you can look at the match_archives option under storage. This change is documented at https://torsion.org/borgmatic/docs/how-to/make-per-application-backups/#code-35