It could very well be that my impression here is highly inaccurate, but both from observing the amount of mods available, and reading complaints from modders in the past, I have gotten the impression that Firaxis has done a lot less to make the two latest games "easy" to mod.
With all due respect: you worded an uninformed opinion about the state of modding very definitively. Time and time again, the most vocal complaints I've read about "the state of Civ modding" come from people who don't know how to mod. I don't mean to attack you, but it's frustrating to read these opinions repeated as fact over and over.
I want it known to the developers that people who actually mod sincerely appreciate the overall flexibility of the modifier system, the general move away from hardcoding stuff (and therefore less of a reliance on unstable Lua), the addition of an asset editor to make 3D graphics, and also the ease of use of mods in multiplayer. The last point is extremely important to me. It's never been easier to set up modded games with your friends.
Civ 5 specifically did receive a lot of complaints if I'm not mistaken, and the amount of mods made for that game was significantly less than for Civ 4 or Civ 3. For Civ 6 I have the impression that there is a little more mods, but not that it is easier to change more complicated things. I've also seen a modder complain than it was easier to mod for Civ 5 than 6, and that is why he or she went back to modding for 5 instead. I don't really know how accurate any of this is, I just know that there are far fewer mods, and much less mods that change game mechanics than before, even though the playerbase has increased a lot.
It's funny you mention the complaints of Civ 5 because most of the uninformed complaints about Civ 6 reference how much better Civ 5 modding was. I suspect no matter what that the same opinions will return with Civ 7, complaining about how much better Civ 6 modding was. Human nature, I guess.
Anyway, the number of mods available is not a good surrogate for moddability, and one anecdote of one unknown person saying "Civ 5 was easier to mod" is not a compelling statement either. What does that even mean, easier? Some things are easier in Civ 6, some things are different. By and large, I think it is actually much easier for a person to make a mod for Civ 6 because of the modifier system. Most stuff in Civ 5 is really hardcoded and requires Lua, which is not as easy to grasp or implement as simple database changes.
Many mods on the workshops are maps anyway. Civ 6's late worldbuilder release contributes to the difference. Even though I don't care to make maps, I know making custom maps is great fun for a lot of people, and I will readily concede the Civ 6 worldbuilder rollout as inferior to that of Civ 5. Took way too much time.
Modding for Civ 3 and Civ 4 was of course very different things. In Civ 3 it was very easy to make a mod or scenario with the editor if you didn't want to add any new graphics, and even if you wanted to do that it was probably not very hard if you read up on what you wanted to do and found suitable artwork created by a fellow modder in the library. This set the bar very low for who could make mods, and therefore there was a huge number of mods and scenarios made by talented people for that game. While there were several things that was hardcoded in the game that you couldn't change with the editor, there was also a huge amount of things that you could change, like technologies, units, stats, governments and more. The Civ 3 editor was a continuation of the great game editors that game companies made in those days like in Heroes of Might and Magic 3 and Starcraft.
For Civ 4 it became a lot more complicated since there wasn't any editor available, or if there was an editor it didn't have many of the features that the one in Civ 3 had, so the bar for mod making was raised higher. But the developers had instead made the game very mod friendly for anyone skilled in Python, and anyone willing to learn it, and this opened up the possibility for much more advanced mod endeavors. So even though the bar was raised a lot higher for who could participate, the civ mod community had its greatest time in this period.
The days of simple editors are gone. Civ 3 is a much simpler game, and was cheaper and easier to make than Civ 5 or Civ 6. And graphics have improved and changed so much, as have standards: you can still make a mod that adds a "building" without any added model, but nowadays players are spoiled for choice by modders who take the time to actually include these things.
It's hardly a fault to Civ 6 that it's more advanced than Civ 3 and thus requires more input from both developers and modders.
Obviously it is still possible to create highly advanced mods for Civ 5 and 6, like the Voc Populi and Anno Domini teams have shown, but it seems to me that it is harder than before. Of course a lot of this can also be attributed to the increased resolution and polygon counts used for artwork in Civ 5 and 6, but I don't but that this is the only reason when there also seems to be fewer mods around that mostly change gameplay. I will take your disagreement into account however.
"To me it seems harder than before" - Odd statement if you've never actually made a mod yourself. And it's not just increased "resolution" or polgyons - it's an increase in overall graphic presentation. In Civ 5 and earlier games, unique buildings didn't even show up on the map--they're just icons in your menu. In Civ 6,
every building is actually represented, which again, raises the standards that mod users have.
---
Again, I don't blame you for having this opinion. It primarily arises from two things. First, it comes from Civ 5 players observing the "lack of a Vox Populi"/"lack of a Rhyse and Fall"/whatever for Civ 6. Second, it comes from a small handful of vocal Civ 6 modders who are upset they can't make their huge overhaul mods becuase the DLL hasn't been released. Other users see these complaints and it becomes a game of telephone that we all repeat. These are pretty narrow complaints that miss the forest for the trees.