RestSharp doesn't encode query parameters when it is told not to. For example, this simple test passes:

    [Fact]
    public void Should_not_encode_pipe() {
        var client = new RestClient("http://example.com");
        var request = new RestRequest("resource");
        request.AddQueryParameter("ids", "in:001|116", false);
        
        var actual = client.BuildUri(request);
        var expected = new Uri("http://example.com/resource?ids=in:001|116");
        actual.Should().Be(expected);
    }

However, what I found now is that the Uri object encodes the pipe character. So, the actual URI values in both expected and actual vars in the test contain the same string with encoded pipe char, where the original URL string property for both of them contain non-encoded values.

I fixe in my PR but for .NET < 6 the code uses an obsolete Uri ctor. It works, just hoping they won't remove it.

Sorry, closed prematurely. It still has issues with how some characters are encoded, it's different per framework. Will look at it again soon.