Two issues with this question.
Firstly, Elton John has had Top Ten hits in every decade since the 1970s, but he didn't have any UK No. 1s in either the 80s or the 2010s; and secondly, he's not even the first person to have Top Ten hits in six decades. Cliff Richard had Top Ten hits in every decade from the 1950s to the 2000s (Noughties).
Elton John did have a US No. 1 in 1985, with That's What Friends Are For, which only reached No. 16 in the UK. But his last No. 1 on the Billboard Hot 100 was Something In the Way You Look Tonight / Candle In the Wind 1997. His last appearance in the Hot 100 was in 2000, with Someday Out of the Blue, which made it to No. 49. Written and recorded for the DreamWorks animated film The Road to El Dorado, this one failed to make the charts in the UK.
So no No. 1s on either side of the Atlantic in the 2010s – and even in the UK, he only just made it into the Top Ten. His only entry was Step Into Christmas, which was originally released in 1973 but reached No. 8 in 2019. (This is not mentioned on Wikipedia's Elton John singles discography page, but it is on the Step Into Christmas page.)
Full details of his UK Top Ten entries are in this article on the Music News website, reporting the six–decade achievement.
Elton John's first No. 1 hit was (famously) Don't Go Breaking My Heart, with Kiki Dee, in 1976; his second, and his first on his own, was (equally famously) Sacrifice / Healing Hands, which entered the charts in June 1990. (I hadn't been aware of this until now, but both Sacrifice and Healing Hands were issued separately as singles in 1989, when they only reached Nos. 45 and 55 respectively.)
He had two more UK No. 1s in the 1990s (Don't Let the Sun Go Down On Me, and the aforementioned Princess Diana eulogy) and two in the Noughties (Sorry Seems to Be the Hardest Word in 2002 with Blue, and Are You Ready for Love in 2003, with Ashley Beedle). But for his next UK No. 1 he had to wait until 2021, when two came along almost at once: Cold Heart (with Dua Lipa and Pnau), and Merry Christmas (with Ed Sheeran).
So the question should have been more like " Which recording artiste has had UK Top Ten hits in every decade from the 1970s to the 2020s?"
As mentioned in the opening paragraph above, however, Elton John wasn't even the first person to have UK Top Ten hits in six consecutive decades. Cliff Richard first made the Top Ten in September 1958 with Move It! He had four more Top Ten hits in the 50s, 33 in the 60s, 4 in the 70s, 15 in the 80s, 7 in the 90s and 4 in the Noughties. His last was Thank You for a Lifetime in September 2008. This was a celebration of his 50–year pop music career and a tribute to his fans, including the line "I gave you a lifetime, you gave me the time of my life".
The Grauniad has an article on its website celebrating Elton John's six–decade achievement. But a paragraph at the end announces "This article was amended on 15 October 2021. Cliff Richard has also had Top 10 hits across six decades if releases with his groups the Drifters and the Shadows are counted; the headline, subheading and text has been changed to reflect that Elton John's releases have been as a solo artist."
Either the Drifters or the Shadows (which were of course the same group) were credited on Cliff Richard's first 17 hits (all but four of which made the Top Ten) – the Drifters on the first six, up to July 1959, and the Shadows thereafter. After that it gets complicated, but his association with the Shadows continued off and on until at least 1968. Personally, however, I'm not convinced that the Peter Pan of Pop's use of a backing band disqualifies him from being described as a solo artist. How many of Elton John's hits feature just him and his piano?
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