This rule should not exist. I live in 2015 where dialup users are a minority.
There are more than one admin and moderator in Urho3D forums. Those initial forum rules were drafted without any formal discussion with all the stakeholders. Personally, I don’t care much when the animated GIFs does not distract too much of my attention. I don’t think server load or network bandwidth are the main concerns here.
The rules were probably (at least partially) copy-pasted from somewhere and the animated signature rule was never enforced, as far as I know. I’ll edit the rules in spirit of what weitjong said.
Thanks! I can’t live without Taylor Swift <3