Npgsql version 11.0 is in development.
Note
We're considering to start dropping support for synchronous APIs (NpgsqlConnection.Open, NpgsqlCommand.ExecuteNonQuery, etc.) starting with Npgsql 11.0. The current plan is to deprecate the API by throwing a runtime exception by default (with a switch to re-enable synchronous I/O) for Npgsql 11.0, while possibly completely removing it for Npgsql 12.0. This is in line with ASP.NET Core and .NET APIs in general, which are moving in the direction of async I/O only (for example, System.IO.Pipelines doesn't have synchronous I/O). If you have any questions or want to share you experience/issues with async I/O, please feel free to post in the issue.
Previous versions of Npgsql supported a high-performance mode called "multiplexing". While this mode could increase throughput in certain extremely-high-performance scenarios, it did not scale in highly-concurrent situations with many CPU cores, and added quite a bit of complexity to the codebase. We'll evaluate re-introducing a similar implementation that's more scalable and better-designed.
Npgsql 11.0 removes some APIs to support PostgreSQL large objects; these were obsolete since Npgsql 8.0. The APIs were old and had some design issues, and were also not necessary, as the corresponding PostgreSQL functions can simply be called directly.