Chiming in with my experience…
The AI did improve…barely. Not enough to matter. I’ve been playing on Immortal.
It can finally take a walled city, IF it outnumbers you 6 to 1. And I don’t mean a 6 to 1 ratio, I mean only 6 units against 1. If it’s any more units than that, it will run into a traffic jam and have no clue how to herd all those units to overwhelm you.
The AI’s one and only shot at beating you is still in the very early first 20-40 turn rush, before walls, before you have a chance to match its massive starting army advantage. That’s it. That’s the AI’s only hope. If it can’t beat you in the first 50 turns then it’s never going to beat you.
That means the entire game past turn 50 is pointless. You will win. It’s just a matter of time. It still can’t win via domination. It still can’t win with space race because the requirements are so ridiculous and the costly space port districts so easy to take out. It still has no idea how to take advantage of tourism bonuses to generate enough to win a culture victory. It can’t win a religious victory because you just declare war and wipe out its missionaries the instant they threaten to convert you.
I’m not even an expert at this game. I was never able to win any Civ 4 games beyond Monarch difficulty, yet I’ve never lost a Civ 6 game on Immortal (barring the super early rush). I’ve never been in the slightest danger of losing to any of the victory conditions. The AI just completely lacks the ability for the final push needed for culture or scientific victories, and a human player will never lose to a religious victory.