David Engel has an announcement:
We announced Microsoft.Data.SqlClient in the first half of 2019 and shipped the first stable package later that year. That release coincided with .NET Core 3.0. We’ve been developing Microsoft.Data.SqlClient in the dotnet/SqlClient repo since that time, over the last five years. It’s now time to start the deprecation process for the System.Data.SqlClient package. We plan to take a staged process to deprecation, with the intent to align support changes and associated code transition activities between these libraries to natural migration points between .NET major versions.
I haven’t used System.Data.SqlClient in a while, so I’m not sure how much is in that library that isn’t in Microsoft.Data.SqlClient.