I think Dictionary implements IReadOnlyDictionary in recent framework versions - I don't think the creation of the ReadOnlyDictionary should be needed here.

Ah yes, that's true. It's one of those strange things where a writable dictionary actually implements read-only dictionary ....

I'm not sure what the "best practices" are in that regard, but I guess in internal classes there is no need to be a control-freak :)

I'll change that.

Also here, if we don't create a ReadOnlyDictionary explicitly, we can get this back to a one-liner and avoid the additional method.

(Nitpick - the codebase doesn't use explicit internal modifiers elsewhere.)

Actually the method exists for the sake of tests ... but I don't know if the test

+        [Fact]
 +        public void IrrelevantKeysAreIgnored()
 +        {
 +            var irrelevantSettings = new Dictionary<string, string>()
 +            {
 +                ["whatever:foo:bar"] = "willBeIgnored",
 +                ["irrelevant"] = "willBeIgnored",
 +            };
 +
 +            var directives = KeyValuePairSettings.ExtractDirectives(irrelevantSettings);
 +
 +            Assert.False(directives.Any());
 +        }

provides much value ...

(about internal : dammit, I thought I had removed them all !)

Makes sense, I didn't notice the test dependency, thanks 👍