As someone who has been experimenting with various mods (art and code), I have to say that I fully agree with you here, feas.
The guidelines you have in mind are relatively strict and aim to sanction cheaters or client mods that alter the overall style of the game experience:
Modifications to the client content (graphics, sounds, ...) are not allowed by PS License. In particular we will not tolerate players using mods to connect to an official server if those allow cheating, or modify the visual aspects of the game or its design.
In practice, I have seen the developers are relatively tolerant with client side modifications, unless they are incompatible with the "spirit" of the project:
We encourage you to experiment with mods and changes to either our source code and to our art assets. PlaneShift is an open source project, based on community contributions, and made to ensure people can learn from what we made, and we can learn from them.
So, to answer your question about spell effects that freeze a client, I'd say it is exactly the opposite: If you were not allowed to disable them, that would be an unfair advantage for players with better hardware