I thought it was not .NET 4, but just quietly removed in new versions (so even .NET 4 can’t get current).
I’ve cited the detailed history of this in forum links. For a contemporary alternative, there is possibly this:
https://github.com/utelle/SQLite3MultipleCiphers-NuGet
System.Data.SQLite: RC4
Supported for compatibility with earlier System.Data.SQLite versions only. Don’t use it in new projects. Since early 2020 the official System.Data.SQLite distribution no longer includes the RC4 encryption extension.
If it has any future value, especially as Duplicati Inc. gets in new areas, maybe it could scan less than all.