From memory, about 50% of the users visible in Matrix are on the matrix.org server, which is one of about 30K deployments total that we're aware of, directly or indirectly. This will likely change in the near future as the French deployment expands, which should dwarf the matrix.org server. In the medium/long term, we'd like to at least disable registration on the matrix.org server if not turn it off entirely (once decentralised accounts land).

The identity server is used ONLY for mapping email addresses and phone numbers to matrix IDs, and is strictly optional, so I'm not sure it counts as centralisation. We are also hoping to remove it entirely from the mix in future (switching to storing 3pid->mxid mappings per room instead).