the glitched seeds started with the same update... the seeds were probably a side effect of the farlands being removed. the y coordinate one which you didnt know about not working is because the code removing the farlands doesn't apply to the y coordinate. also removing the seeds would require revising the code if that even is possible without causing the farlands to reappear so start with the sky farlands. (Y seed: 240144965573432) also i once modded my game (with cubic chunks) to try to get it to work but only could get the effect once. (off topic)

Main Section: Notch forgot to remove the sky farlands. This can be seen by when you use the customized world generation code antvenom discovered that can bring the farlands to spawn by multiplying the coordinateScale which he used to move the farlands. he found that the sky farlands which involves the same thing only in the heightScale. i noticed it can be done in vanilla minecraft. meaning its technically a glitch. if you follow his instructions he left in the video you can repeat this.  (i cant believe nobody reported this. its been over a year since the video)

Video: https://www.youtube.com/watch?v=h-rlN9J_EVI