Transfers have returned! See this post for more info: https://na.leagueoflegends.com/en/news/game-updates/features/account-transfers-half-limited-time
There is still a bit more work that needs to be done to get the service usable again, and the team is hoping to have it back up and running by late January/early February.
Also - when the service does come back up we are planning on offering transfers at a discounted rate for a couple weeks. There will be a news post when we have more information on exact timing, so keep an eye out for that!
We originally announced that server transfers would come back online this week, but we hit a snag and have to put the date back. We ran into some issues with how our data centers communicate with each other, and our engineers estimate it will take at least 5 to 6 weeks to be fixed. We're really sorry about this.
We'll update you when we have more news.