WoW is interesting because they’ve turned the weather on and off countless times, and iterated on it in many ways. It’s actually on, working, awesome and dynamic right now, in Legion (I know because I’m playing atm!), but before that I think the only time it was working this well and this dynamic was… I want to say late TBC…
It’s particularly interesting because it’s so subliminal, strangely, that people don’t always notice when it’s present/gone. You get like one guy (probably me) going “Why are there no awesome storms in Stranglethorn Vale any more?” and no-one responds… But they’re back right now, certainly in the Legion zones. We got snow, fog, varying kinds of and intensities of rain and so on.
Re: day/night, you’re correct, they have never turned it off. As for immersion-enhancing, personally I respectfully disagree, but I definitely can see how one might feel that way - it sort of connects our reality to theirs. When I played heavily though, and had to play at night hours, so always only saw night in WoW, I found it sort of sad, myself. My preference is for a short-night long-day cycle - GW2 does this, but also makes the night so light you can barely tell, which kind of ruins the entire point imo.
You’re also correct re: darkness. WoW has gone back and forth on this. Early WoW was reasonably dark, it certainly wasn’t excessive or “realistic” in the least (I actually like games that get darker myself - DAoC had a good balance - EQ went too far in favour of ultra-darkness imo), but due to complaints - presumably from people like me, who always played at night - they lightened up the darkness until it was close to daylight in most zones, just a bit more blue. Then as you say, with Cataclysm, they experimented with “Making Darkness Dark Again” as we might say these days, but yep, it didn’t stick. If say, early WoW was a 5 on 1-10 of dark to light at night, and TBC/WotLK-era was say, a 9 (where 10 is full daylight), post-Cata has mostly been a 8, with a 7 in some zones.
However, I just found out (when Googling to see if I could find good comparison pictures - I couldn’t ), there is a potion for “freaks like me” - https://www.youtube.com/watch?v=iUJEG4xP7SU
I had the voiceover off so apologies if it’s awful but the effect is clearly shown. It’s rather darker than the original (more like what I’d call a 3-4 - akin to DAoC, so kind of ideal for me!), but it’s a cool effect. I mean, I think it should be a graphical toggle, rather than a potion, but hey, at least it’s something.
Re: dynamic weather generally, I absolutely adore it, and always mod in improved dynamic weather to games where I can - Skyrim and FO4 for example.