hi. It has been quite a while when I encountered this.
What i noticed was the factory default firmware didn't have this recents key, however once firmware is upgraded this key comes in and can't be removed from SKT.
so only option I had was to keep the phones at factory default and not upgrade the FW.
Thank for you answer, to bad that Cisco implement features which suddenly is mandatory and actually have quite a lot of impact for customers. My customer have said that they just don't want that key, they want the layout as it was before the upgrade.
I know that this a year old question but now from firmware 11.5 you can disable the recents softkey and avoid this problem.
here is an article that explains how to do it.
How to hide the "Recents" softkey on 7800 series phones
There it is. Last point of the phone configuration tab. You can bulk "disable" them.