The promoters seem to be dancing around "first" a bit and Elton did some stuff 3-4 decades ago that might apply, but nothing is clear cut."His appearance on the festival’s What Stage will not only be his first Bonnaroo appearance but also his first appearance at any modern United States music festival."
He guested, presumably unbilled, at this festival.
Elton guested seperated with the Doobies and The Eagles at San Fran's "Day at the Green" in 1975.
Elton guested with Kiki Dee at the Central Park "Dr. Pepper Music Festival" in August 1977.
5 songs at I Heart Radio several months ago.
He did 45 minutes at Bridge School in 1992 that's on youtube. 2010 as well.
Ignoring those Elton appears to have played in Baltimore at something called "Painter's Mill Music Fair" in 1971.
"Tree Frog Productions had Elton John open for Leon Russell, Derek and the Dominoes, Cheech and Chong, Emerson Lake and Palmer and many more in '70 and '71.(3) "
Painter's Mill Elton concert poster, though this appears to be just a double bill.