It was a major ship for about 3-4 years but lost steam over time as most ships do as people drift to other, newer fandoms. It experienced flare-ups with each new short, especially Frozen Fever, but that was in 2015, and the fandom had quieted down quite a bit by the time Olaf's Frozen Adventure came out in 2017.
No, I think the real issue is what happened between OFA and Frozen 2 (2019): the Great Nipple Ban of 2018. It's not that puritanism suddenly started taking off here, it was always here, but a ton of nsfw artists and their audiences left after the Nipple Ban, and that left the puritans as a larger slice of what was left afterwards without a single bit of actual growth.
Suddenly there were far fewer sex positive fans who understood that there's no harm in shipping 100% fictional adult sisters, and so the puritans seized the opportunity to be disruptive and make those who were still here feel unwelcome. Sure, Frozen 2 gave us a new character to ship Elsa with, but we already had Marisol, Blue Dress Lady, and all the myriad other Disney princesses, and yet Elsanna remained the most popular ship.
So yeah, tl;dr - I suspect the Great Nipple Ban was responsible more than anything else