I was able to test out this theory with the following custom interfaces and by simply dropping the new interfaces dll to the publish folder without recompiling the consuming code, the IMyReadOnlyList<T> interface was automatically supported without breaking the code.

Original Interfaces DLL code

namespace InterfaceTest
{
    public interface IMyReadOnlyList<T>
    {
        int Count { get; }
        T this[int index] { get; }
    }

    public interface IMyList<T>
    {
        int Count { get; }
        T this[int index] { get; set; }
    }
}

New Interfaces DLL code

namespace InterfaceTest
{
    public interface IMyReadOnlyList<T>
    {
        int Count { get; }
        T this[int index] { get; }
    }

    public interface IMyList<T> : IMyReadOnlyList<T>
    {
        new int Count { get; }
        new T this[int index] { get; set; }
        int IMyReadOnlyList<T>.Count => Count;
        T IMyReadOnlyList<T>.this[int index] => this[index];
    }
}

Consuming Code

using System;
using System.Collections.Generic;

namespace InterfaceTest
{
    class Program
    {
        static void Main()
        {
            var myList = new MyList<int>();
            Console.WriteLine($"MyList<int>.Count: {myList.Count}");
            Console.WriteLine($"IMyList<int>.Count: {((IMyList<int>)myList).Count}");
            Console.WriteLine($"IMyReadOnlyList<int>.Count: {(myList as IMyReadOnlyList<int>)?.Count}");
            Console.WriteLine($"MyList<int>[1]: {myList[1]}");
            Console.WriteLine($"IMyList<int>[1]: {((IMyList<int>)myList)[1]}");
            Console.WriteLine($"IMyReadOnlyList<int>[1]: {(myList as IMyReadOnlyList<int>)?[1]}");
        }
    }

    public class MyList<T> : IMyList<T>
    {
        private readonly List<T> _list = new List<T> { default, default };

        public T this[int index] { get => _list[index]; set => _list[index] = value; }

        public int Count => _list.Count;
    }
}

Original Output

MyList<int>.Count: 2
IMyList<int>.Count: 2
IMyReadOnlyList<int>.Count:
MyList<int>[1]: 0
IMyList<int>[1]: 0
IMyReadOnlyList<int>[1]:

New Output

MyList<int>.Count: 2
IMyList<int>.Count: 2
IMyReadOnlyList<int>.Count: 2
MyList<int>[1]: 0
IMyList<int>[1]: 0
IMyReadOnlyList<int>[1]: 0

Make IList<T> inherit from IReadonlyList<T> #16151

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