Are U.S. special delivery stamps valid for postage?

The last four U.S. special delivery stamps were issued from 1954 to 1971.

American Credo Issue National Postal Museum

U.S. STAMPS & POSTAL HISTORY

Royal Mail warns there are only 100 days left to use stamps without barcodes, Royal Mail

Are old US postage stamps still valid to mail things with? - Quora

Special Delivery Stamps - Guide to Value, Marks, History

Special Delivery Stamps - Guide to Value, Marks, History

Fourth Bureau Issue

Daniel F. Kelleher Auctions, LLC Sale - 722 Page 56

Special Delivery Stamps - Guide to Value, Marks, History