This champion was drowned in nerfs.
Tanks and bruisers have been in a strong spot since the release of the preseason on Nov. 16, 2022, mainly due to the League of Legends team introducing and buffing tank items across the board. Unsurprisingly, tank items and some champions were targeted with heavy nerfs coming into the first patch of the season and these nerfs left a toll on this champion specifically.
Coming into Patch 13.1, Rammus was comfortably sitting at a 54 percent win rate across all ranks, thanks to Riots large-scale buffs to tank items. After Patch 13.1 dropped, Rammus now has a win rate of only 45.45 percent, according to stats site U.GG.
The most likely reason why Rammus win rate plummeted by almost 10 percent is that the champion received nerfs to his base stats with base attack damage getting decreased from 56 to 53 and base HP going from 634 to 614. Aside from that, Defensive Ball Curl now gives only 25 armor, far less percent armor, and Riot tweaked the interaction between JakSho the Protean and Rammus Defensive Ball Curl so the two interact more cleanly with one another.
Riot also nerfed JakShos interaction with minions and monsters, with Voidborne Resilience now not applying to minions and monsters. Finally, Legend: Tenacity got its Tenacity reduced from 30 percent to 20 percent.
Bearing in mind Riot nerfed Rammus favorite Mythic item and Tenacity, it seems now the devs overdid it with nerfs for Rammus and the champion will now need some love to get back in the game.