When WIll Yorick Rework Take Place?

Soon (TM)

what is a yorick Kappa

They have probably already started, but there are a lot of things that need to happen in order for it to be fully worked out. First there will be concepting (which is probably already done at this point), then figuring out his kit overall (also hopefully already done at this point), then the long stuff of doing the model and animations (this stuff can take awhile as there is modeling, rigging, figuring out the animations, syncing the animations to the coding of the skills and so on), and coding which also is a lot of hard work. With a team working on it, day and night, at least a couple months worth of work in just the last few phases.

So to say it's probably the end of the year may be a bit far off, but with everything else they want to get done as well. . . probably October at the earliest. Learn to play him without a tear for now, because hopefully they will remove his full reliance on the item and make him a more usual juggernaut build instead (or give him a gap closer so he can play as an initiator).

If memory serves me right, the mage class rework will be done before yorick.

Taric (who is almost finished) and the mage update are first. It was said if not something totally terrible happens, Yorick Mori should be remembered at the end of the year (he should hit PBE then). I think they will start right after the mage update or maybe even before.

https://support.riotgames.com/hc/en-us/articles/202294884-Champion-Update-Schedule

This listing always shows a rough outline of which update is coming next. Taric is up first, followed by the Mages and then Yorick. Taric's will be fairly soon, with the Mage update said to be more in mid-year, so Yorick is likely to be near the end of the year at earliest.