I see no suggestion in the New Testament that tithing is purely a ceremonial law that ended with the coming of Christ and the sending of the Spirit on the day of Pentecost. Tithing is operative in the New Testament economy.
As New Testament believers, we have seen Christ from the view of fulfillment rather than the view of promise, as would have been the case in the Old Testament. Since we have seen the love of God for us in Christ in a way that Old Testament believers could only anticipate through foreshadowing, there is an argument that New Testament believers ought to be more generous than simply 10% of their income.
I’m not convinced that it is an example of legalism to suggest that tithing is mandatory in the New Testament. I think that one who has experienced the love of God in Christ would desire to tithe. It would be my natural response to an overwhelming expression of God’s love for me in the gospel.