Dragon Ball Super has completely forgotten about Gotenks, the fused form of Goten and Trunks. Towards the end of the original Dragon Ball story, Akira Toriyama introduced Goku's second son, Goten, born while the protagonist was consigned to the afterlife. Around the same time, Vegeta and Bulma have a son of their own, Trunks, a future version of whom had already featured in the Android saga. The young Goten and Trunks form a mostly comedic duo at first, but it's shockingly revealed that both kids have secretly already unlocked their Super Saiyan transformations.
When Majin Buu, the final villain of Dragon Ball Z first appears, Goku and the Z Warriors quickly decide that Goten and Trunks are Earth's best hope against this powerful, pink enemy and Goku imparts a secret technique known as fusion to enhance the boys' powers. Under Piccolo's tutelage, Goten and Trunks become Gotenks, the second Super Saiyan 3 in the series and a fighter of terrifying power. Although it was ultimately left to Goku to take down Buu for good, Gotenks put up a strong showing and might've defeated the villain had he possessed a little more maturity and tactical nous.
Fans might've expected a more experienced Gotenks would feature prominently in sequel series Dragon Ball Super, but that hasn't been the case, with Goten and Trunks lucky to be included, let alone involved in a fight. Individually, Goten and Trunks are incredibly powerful fighters, perhaps even overtaking Gohan after he became so rusty he could barely transform, but their fused form would be second only to Goku and Vegeta in power. Despite this, Gotenks is metaphorically buried by Beerus in the first arc of Dragon Ball Super and then has a brief fight of no real consequence in the Ressurection F story before running away and hiding from Frieza. Aside from a later appearance in filler episodes, Goten and Trunks do little else in the series.
Despite the strength of both youngsters, and especially their combined form as Gotenks, the duo are deemed too young to compete in the Tournament of Power or any of the other inter-universe clashes, and the boys eventually start being sent to protect Android 17's island whenever there's a big fight brewing, keeping them entirely out of the way. This is especially strange since the likes of Krillin and Tien are selected to represent Universe 7 in the Tournament of Power, despite being far weaker than Goten and Trunks. Furthermore, Kale and Caulifla demonstrated exactly how useful fusion is in the tournament setup. More confusingly, the fight against Moro sees even Yamcha and Chiaotzu dragged out to fight against the villain's goons, while Goten and Trunks are once again relegated to protecting Android 17's island. That's like picking Hawkeye in a fight when Thor was begging to be tagged in.
It's quite clear that Dragon Ball Super is deliberately finding ways to keep Goten and Trunks out of the picture as much as possible, whether by deeming them too young, or by assigning them to off-screen missions. This is a complete retcon in the sense that Goku and the gang were quite content to have the boys fight Buu in Dragon Ball Z, so there's no reason for them to start playing the concerned grown-ups now. Gotenks isn't the only retcon in the sequel, of course, with Potara fusion rules, Gohan's power and Saiyan hair growth all being rewritten.
As for why Gotenks has been only a peripheral figure in Dragon Ball Super, it's possible that the fused fighter's complete lack of seriousness forced the adults into thinking they couldn't rely on the duo in future battles. In real-world terms, however, not all fans enjoyed Gotenks' goofy fighting techniques and awkward humor in Dragon Ball Z, and his lack of opportunities in Dragon Ball Super is perhaps a direct result of not being especially popular compared to the classic characters.