Pegasus flights are still bookable from Stansted on their own website and the Stansted website as well. No results on the Luton website and nothing mentioning Luton on the Pegasus website.
Somebody has put Pegasus on the Luton Wikipedia page but with no links or references to support it. The fact that whoever it is has put Ankara as a destination as well makes it seem even more questionable. Unless of course this relates to somebody who has seen this on here and decided to change it on Wikipedia anyway.
I'm not saying it won't happen. It wouldn't be the first airline to leave Stansted and move elsewhere but could I ask Boeing737-8 where you have heard about this??