It is compatible. I have Graylog 5.2.1 along with Sidecar agent 1.5.0.
What most matter is the version of winlogbeat for example (if you are using it).
I don’t know why they added Winlogbeat 8.9.0 in the Graylog sidecar 1.5.0 package but it does not work for me with OpenSearch 2.11.0.
So I replaced the Winlogbeat 8.9.0 by the 7.17.13 (works for me). You should try the recommanded version for Opensearch version though (7.12.1)
If you use NXLOGs I think the latest version works as it use GELF.