To add to this, this won't necessarily mean we won't get Halloween type stuff at Halloween or similar stuff at other holidays. Looking at it it's more like this:
"Narrative Team": Hey guys, we're making some lore stuff about what Snowdown's like in Runeterra, specifically around Freljord! Take a look!
"Skins Team": OMG INSPIRATION. Guys, let's make a bunch of Freljord style skins for Snowdown!
"Featured Game Mode Team": You're not leaving us out are you guys? Now we wanna put out a new game mode for Snowdown!
So the teams are able to more flexibly do work and act both more independently while also being more cohesive. When one team wants to do something others can join in, so if one team's doing christmas stuff for Snowdown, others can also pop in and help. Before it was almost a criteria thing, where they needed X, Y and Z for the event specifically, which for holidays has mostly always been "Featured Game Mode", Skins and Summoner Icons.
To kinda put the Harrowing thing in perspective; Harrowing was effectively League's Halloween before. Because of the lore they've now made for it this is no longer the case. This doesn't mean that there'll never be a League Halloween celebration though.