As an architect
I want to more options for node count of my clusters beyond just multiples of 3
so that I can optimize my costs

Since OpenSearch handles replication at the index level and not at the node level, you don't strictly need multiples of 3 to provide a highly available cluster. Also most OpenSearch users use a replication factor of 1 so you don't have 3 copies of the data to spread across 3 AZs anyways.

Supporting more variety in node counts in a cluster would help users coming from a self-managed or alternative OpenSearch provider that have a different footprint and might see their cost go up significantly because they have to deploy a larger cluster than they're used to. Supporting dedicated master nodes would help.