Some Buffalo soldiers served in WW1. How many, I don't know.
My cousin-in-law Roosevelt Jackson was in Fort Huachuca with the 10th Cavalry after he left Wilberforce College. He was promoted to 2nd Lt.and was reassigned or volunteered to serve in the 15th New York, National Guard Regiment which was renamed the 369th Infantry Regiment in France and was assigned to the French Army's 161st Division. He served with a friend of his, from the "Old M Street High School in Washington, D.C., Sgt. Charles Shorter. I have pictures of both,in uniform.
In my opinion the real "Buffalo Soldiers" were phased out at the beginning of WW2. In WW2 they called the 92nd Div. "Buffalo Soldiers" and gave them some of the worst sectors, without guns and ammunition until the last minute, but they were not the originals.
I landed in Korea in January 1951, when the North Koreans and the Chinese Armies had encircled the "Pusan Perimeter" and a cousin of mine , Kenneth Shackelford was in the 24th Regiment which was the last of the all Black regiments. They were disbanded in 1952. My all black D.C.N.G. Company was also integrated in late 51-to early 1952.
I agree with Bennie, that the "Buffalo Soldiers" were deactivated at the start of WW2. I can't remember any of the 24th in Korea calling themselves "Buffalo Soldiers",and rightly so!