Heck, I would say that custom zombies contributed a lot to why CoD zombies is so popular in the series. I cannot thank both the modding scene and Treyarch enough for these custom content. I'd attribute the continuous development and thrive of custom zombies content supported by the modding scene, to the number one reason of why we have Black Ops 3 modtools now, which is awesome in its own rights. Custom maps in the period of 2013-2015 were diversed in that some are of top notch quality, some are funny maps memes and some are of course box maps. You could just get zombie chronicles for BO3 if you have a PS4/XB1, all the WaW maps remastered in HD and they are stunning.
Unless you get the pc version, no, the map packs aren't free unfortunately. You can see a lot of custom maps now have weapons from other CoD titles or other games, rather than vanilla WaW weapons plus like 42 perks etc. In other words, no you cannot get them for free.
Since then, some really good modders created tools for the community to enhance the creation of custom content and therefore more custom maps come with Treyarch zombies features, from weapons to perks to models etc.
I think it is due to YouTube when some channels were featuring WaW custom maps in their gameplay during the 'down time' of getting no zombies content from non-Treyarch CoD titles XD, or they just wanted to play more zombies.
Slowly towards 2012, UGX Mods has an influx of enthusiast newcomers interested in modding and a spike of custom maps/content appeared soon after. IIRC again, in 2011, some members of one of the bigger custom zombies forum - ZombiesModding went into some quarries (don't remember exact why.) and the forum began to decline in population after a team of modders started a new forum - UGX Mods, which is still the more, if not the most, popular custom zombies forum today. The community kind of peaked off since the release of MW2 but it was never dying by any means. The modding community gained traction after the modtools were finally updated to include contents in all DLC packs, this was around September/October of 2009 I think.
This was when more people were interested in creating their own custom maps and a decent amount of custom maps (now classics) were made.
Treyarch began to update the modtools with DLC content as their game cycle went on custom zombies maps can then include DLC maps' features such as perk machines, traps, dog rounds, Wunderwaffe DG-2 etc. IIRC WaW custom zombies started with a bunch of devoted modders who have been playing around with the modtools, having only the content of Nacht to begin with (which means that the custom zombies maps only have barebones features, as in Nacht). Below is a brief history of WaW custom zombies (from 2008) as I recall: The WaW modding community started with some dedicated modders from CoD4 or other game, and it grows to this thriving scene that is today compared to BO3 modding, we now have talented and skilled modders to experiment and create content with the knowledge of overall modding and 'how zombies works' if that make sense I think we'll see a steady growth of quality zombies maps in the workshop as long as the scene is not dead. Is the giant free on Black Ops 3 What perks are on Nacht der Untoten Which Black Ops 3 Zombies map is the best Does Black Ops 3 only have 1 zombie map.