Why not support SequenceReader

[oven425]

SequenceReader can read string, improve work
but not use in net472, can explanation reason?
thank you

[ericstj]

SequenceReader was introduced after the System.Memory package was shipped and placed in servicing. The type was only added to .NETCore 3.0 and later. Once we ship an OOB like this that forwards types into the framework/corelib we do not add more API to the OOB. Doing so constrains API development in the latest framework due to restrictions around adding members to existing types / supporting different type locations across different versions, etc. You are free to take a copy of the SequenceReader source and use it internally on .NET Framework.

At this point we aren't taking feature requests / new API to these packages. We're only building them to support security updates.

cc @dotnet/area-system-memory @jkotas @stephentoub