Comments are now revisionable

It is now possible to create new revisions of comment entities. Comments can now be moderated with Content Moderation and they now have revision_user, revision_created, revision_log_message and revision_id fields.

API-First impact

The new fields may impact API clients, because these fields will now also appear in the normalizations. This affects core REST (all formats, also custom ones), and contrib modules like https://www.drupal.org/project/jsonapi and https://www.drupal.org/project/graphql. None of these fields are required, so existing bodies for PATCH and POST requests will continue to work just fine. But the GET, PATCH and POST response bodies are all affected.
(Note that most API clients do not have hardcoded expectations around the complete set of fields that are present — as long as the fields that they care about are present, they work fine. This is also the recommended approach, per https://en.wikipedia.org/wiki/Robustness_principle. Still, this may break some API clients.)

Default content impact

It also affects modules not related to APIs, but that do use normalized entities, such as https://www.drupal.org/project/default_content. Existing exported content will continue to work fine. Re-exported content and newly exported content will contain these additional fields.

Custom database queries impact

Also if there are custom or contrib modules querying the database directly (rather than using Entity Queries) we now have comment_revision and comment_field_revision tables.