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

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

E6 - 1902 10c Special Delivery - DL watermark, ultra marine - Mystic Stamp Company

Daniel F. Kelleher Auctions, LLC Sale - 680 Page 5

Special Delivery Stamps - Guide to Value, Marks, History

U.S. Special Delivery (postal service) - Wikipedia

E12-19 - Special Delivery Stamps (8) - Mystic Stamp Company

Stamp Market Tips: Watch for popular Spanish special delivery stamp featuring a locomotive

U.S. 1936 Great Seal airmail special delivery stamp: Tip of the Week

Robert A. Siegel Auction Galleries, Inc. Sale - 968B Page 51

US postage stamp and shipping prices go up Sunday - Good Morning America

Daniel F. Kelleher Auctions, LLC Sale - 778 Page 19

US Special Delivery Stamps 1885-1971

Ron Leith Auctions Sale - 53 Page 73

E13 - 1925 15c Flat Plate Perf 11 - Mystic Stamp Company