So for the most part, all you need to know is in the title. Paid mods are now gone from the steam workshop. You can read the official statement from Valve here
What’s more important though is the discussion that went on here. If you take a look at that link, what you’ll find is a pretty large thread in the gaming sub-reddit lead by Gabe Newell (The CEO of valve) himself. What it goes into is an intense discussion of the pros and cons of Valve’s attempt at making a system where people can pay for mods.
And what it seems to come down to primarily is an ideological issue of copyrights. I included the “ideological” part because it isn’t quite an issue of copyright law itself, as it is an issue of trying to actually enforcing copyrights.
One of the questions that occurred to some users was the question of determining whether a mod qualifies as a derivative work or not. What would stop people from taking mods for games from websites and passing them as their own? While there are a number of changes which qualify a picture as a derivative work, a mod is significantly more complicated, and one mod will not always appear in the same form as another mod; i.e. there’s a significant difference between a mod that makes a gun look like a banana, and a mod that makes a gun shoot bananas. While the two may both be mods, they may both have very little in common.
But beyond questions of practical implementation, the main issue that arose among gamers was the basic idea of changing a system that already works well, and replacing it with something reminiscent of DLCs. Some even went as far as to say that modding is a concept that defines PC gaming as it exists now, and is one of the primary benefits it has over console gaming. For a full list of reasons, you can read this comment here. It gives a good outline of the overarching reasoning that people think paid mods are a dumb idea. And overall, I agree with them. Paid mods are a terrible idea.
In any case, GabeN in all his glory has decided to include a donate button on mod pages for people to give modders money if need be. It allows the modders to set a minimum donation if they’d like.