using custom skins and its 3rd party softwares in custom games

Any time you make a modification to the client in any way, it's "at your own risk." Some of the custom skin programs are currently allowed, others are not, and the use of their software, even in custom games, can result in a permanent suspension of your account. The use of any custom skin program that allows you to recreate or have free access to skins that are basically the same as those that Riot generates and sells is expressly forbidden as well. (No using custom programs to get knockoff legendary skins)

Many of the programs contain additional software, which can have information about other aspects of the game and can give players an edge. This is why those specific programs are expressly bannable. You can check out this link for a post by Wuks, which contains some of the known custom skin programs, and Riot's stance on them. Just be aware that their policy may change as the software developers change the software.

I've seen kshaway use custom skins on his youtube channel, but I think only on the pbe