I’ve been using darktable 4.0 in the last few weeks. I am a new darktable user, and I’m also pretty new to post-processing for that matter.
Should I upgrade to 4.2? Is it decent stable from a new user perspective? And what about the new sigmoid module? Should ai start using it in place of filmic rgb?
A few thoughts from me…
I think 4.2 is just as good as 4.0 in terms of stability, although I’m not 100% sure.
I really like the new sigmoid module, but it’s really up to you whether you want to use it instead of filmic rgb - it’s not meant to be ‘replacement’ as much as an alternative.
I use either, depending on the image. 
BTW the filmic defaults are different in 4.2 - something to bear in mind maybe.
The new highlight reconstruction options (in the same old HLR module) are very good IMO, and are a reason to upgrade, if you have any issues with blown highlights.
If you do upgrade, don’t forget that 4.2 will upgrade the database so you can’t simply revert to 4.0 if you don’t like 4.2. (I think?) Having said that I’m pretty sure it makes an auto back up beforehand, so you could. Of course you could always make a copy of the darktable config/database folder beforehand. I do this occasionally anyway, just in case I mess something up. Sorry, I’m drifting off topic!
Filmic vs. sigmoid: it’s not totally clear to me what’s behind having two modules doing a very similar job, if I understand it right. When choose one or another?
For me, it comes down to what gives the look I want, more easily.
I’ll try to elaborate a bit… you’re absolutely correct in saying that they both do the same job, that being that ‘transformation’ to the final dynamic range which fits on your screen, or print or whatever.
The difference is in how they do it, in terms of results.
I see filmic as being primarily designed to do this in a very neutral way, to allow you to set everything how you like it, elsewhere in darktable, i.e. color balance rgb for color, tone eq for contrast, etc. Without doing anything else the output tends to be quite flat and low in color saturation.
Sigmoid on the other hand, can give results more similar to the out-of-camera jpg, because it has a different method of… um… doing it. Sorry, I’m not really good on the technical side, especially just before I go to bed! 
But when you increase contrast with sigmoid, the color saturation increases as well, which to me seems very natural.
Also with sigmoid, it will let really bright highlights almost appear to blow out, (especially if you increase the contrast, as I often do) so as to preserve the overall contrast, whereas filmic tries to retain them.
Sigmoid works very well in combo with tone EQ to control highlights and this sort of thing.
I often don’t need to add any saturation in color balance rgb when I use sigmoid.
This is all just my thoughts on it, so don’t take my word for it!
I should add that if you switch sigmoid into the ‘rgb ratio’ mode, it behaves much more like filmic. I never use this option.
Edit: Another point is that I often need to add local contrast if I use filmic, but not as often with sigmoid. I put it down to how it handles contrast as I mentioned above.
Hope all this helps a little 
Edit no 2… @jandren the creator of sigmoid could explain much better I think. You could also search for sigmoid on this forum… there’s been some long discussions… if really want to get into it!
Sigmoid has fewer sliders and controls, and handles colours differently. You’ll find a detailed discussion here:
Yes, why not?
Definitely upgrade. I personally mainly use Filmic, but that is more because I am so used to using it. Yes it is very stable. It is well tested before it is released.
Yes you should upgrade. 4.2 has multiple bug fixes from 4.0 and 4.0.1, regardless of the new modules/features.
Not that it is a 1 to 1 relationship but still one things also is that filmic defaults to a contrast setting of 1 and sigmoid starts at 1.5 or 1.6 ish depending on the settings… Using v5 with higher contrast and using the mid-tone saturation slider you can have much of the same effect but overall I think your explanation is fairly representative… if you like the highlights fading to white or blown in a controlled way then you will get that with sigmoid where as filmic can reel them in a bit more as a starting point…
System wise, I’m on macOS Monterey (12.6), and I’m not upgrading to Ventura (13.1) because I heard darktable won’t work on Ventura. Is this still true with v. 4.2?
Correct. It doesn’t run well.
I was never really happy with how filmic handled tonemapping. I’m very happy with sigmoid. If you can’t spot a difference, just pick the one you work the fastest with.
I like 4.2 quite a bit. While editing images I can get to where I want quickier. Also went back to some old images and I like what I can get from them now, and easier, with fewer modules.
If only it supported my (by now) not so new OM-1 out of the box… 
Is say for sure , yes. The nee highlight reconstruction modes make it easy easier to deal with clipped highlights then in 4.0, and sigmoid is not for everyone , but a lot of people do like it for simpler good results compared to filmic (v6).
As far as i know ,there are no controversial deletions or deprecations .
is the guided laplacian still there?
it’s darktable philosophy not to remove stuff that might break older edits - so yes, it’s there
Sigmoid is great. DT Colors are broken for years now. Very complicate to correct via color calibration, color checker, whatever. Example: cornflower, corn poppy. Horrible workflow.
Colors are much better as they are using filmic. Give it a try!
For reference one can use art and try the for “basecurves”. Often, not everytime, colors are more exact.