If by "never" you mean since the start of Civ 6, then yes, I agree. Civs 1 through 5, though, offered very interesting game play and were a challenge for me to beat at the higher levels. Some of them I could never beat.
Civ 6 is different, and I'm glad you're enjoying it. But it is materially different in the quality of challenge presented by the AI. Purposefully so, I believe.
As you say, it's now meant to be exclusively a fun and relaxing game play experience, whereas in the past it included both that option and more challenging game play options for those who were looking for that experience.
I pretty much agree with this, but I have a slightly different take I think.
I've recently been playing some IV again, and it really is striking how much harder it is. There's no question that a significant portion of that is due to the AI. 1UPT has rendered the AI almost useless in wars. The policy card system in VI which offers so much versatility to the human player is too complicated for the AI to optimize. Eurekas and Inspirations, same deal. Et cetera, et cetera. Some of this is inevitable IMO. As Civ gets more complicated, a human player is naturally going to prove more capable at navigating the additional systems. Personally, despite this tendency I don't view "dumbing down" the game to make it more playable for the AI as a good solution.
Which leads me to my broader point (which supports your contention that the change is purposeful) - AI wasn't the only reason the higher levels were so hard in previous versions of Civ. In the original, for instance, on the lowest difficulty your first 6 citizens were content, but on the highest you only got 2 content citizens. Keeping your people happy was much harder if you played on the highest difficulty. You would notice the difference in gameplay even if you spawned on an isolated island and never encountered an AI civ. Contrast this with Civ VI, where you need the same number of amenities to keep your people content whether you're playing on Settler or Deity. Settler-King basically all play like the exact same game; once you catch up and negate the AI's initial bonuses, Emperor, Immortal, and Deity feel more or less the same as well.
While I'm all for improving the AI, I think it makes sense to implement other ways to make the game harder at higher levels. If on Deity your second citizen demanded an amenity to be content, and every citizen after that demanded an additional amenity, the game would get significantly harder even with the exact same AI. The same could be said for escalating unit maintenance costs; there's no reason why a knight has to cost the same amount of gold per turn on Settler and Deity. Heck, you could even give vanilla builders 4 charges on Settler and 2 on Deity if you want. The new natural disasters mechanic basically begs to have a difficulty component (in addition to the severity settings they're going to let us set at the start of the game). There's ample room within the existing mechanics to make Deity much more difficult (and Settler easier, as far as that goes) without any improvement to the AI. These types of changes would also reduce the pressure on the AI to provide a challenge to the human player, allowing it to roleplay more and ideally increase immersion. I'm not sure you could make VI as hard as the previous titles without improving the AI (I doubt it), but you could make it far more difficult than it currently is.
I don't think there's much chance FXS implements any of this. But overall, I think it's an oversimplification to say that Civ VI is easy because of the bad AI; the bad AI makes it easier, but Civ VI is easier because they designed it that way. This bothers me less than other people, as I'm not a great player, but it still bothers me - if VI fails to overtake IV as the best game of the series (my opinion, of course), this will be one of the reasons why.