.NET Core has pretty strong backwards compatibility rules:

https://github.com/dotnet/corefx/blob/master/Documentation/c...

> We generally don't accept change proposals that are in bucket #1.

...

> Bucket 1: Public Contract Clear violation of public contract.

> Examples:

> - Renaming or removing of a public type, member, or parameter

> - Changing the value of a public constant or enum member

> - Sealing a type that wasn't sealed

> - Making a virtual member abstract

> - Adding an interface to the set of base types of an interface

> - Removing a type or interface from the set of base types

> - Changing the return type of a member

> - ...or any other incompatible change to the shape of an API