For Cassandra 5, I'm not aware that there are issues for DataMiner to use it. The reason why we don't yet say we support it is because cassandra-reaper (repair tool in case you have RF > 3) does not yet fully support Cassandra 5 (see below issue ticket on GitHub for more details). A lot of users rely on reaper to ensure the data between the nodes is in sync. From the moment we know reaper fully supports Cassandra 5, we will initiate testing on our side to confirm we are compatible.

GitHub issue on which we are waiting:
Cassandra 5x: removal of deprecated dateof function causes startup failure in cassandra-migration library · Issue #1436 · thelastpickle/cassandra-reaper

For OpenSearch 3.x, I'm not aware if there are any reasons why we are not (yet) compatible with that version. I'll get in touch with our infrastructure team to verify if we already are doing testing with OpenSearch 3.x, so we can update our documentation.