soooooo as it turns out the 1.20.1 fabric version of the mod (I only tested the latest Neoforge release) actually already features an alpha test version with tweaks and changes which do take into account some of the complaints people including me typically have about the mod in its current state, especially concerning the warden kill requirement and the quality of the block textures which have drastically improved.
I am waiting for the update to be out of Alpha to properly cover the newer changes

What is it ?

Deeper and Darker is a mod which aims to answer the question : what is behind the portal-looking structure within the ancient cities ?
It currently only features a world made of sculking caves with a couple of different biomes, mobs and one hidden structure to plunder.

What does it offer ?

A whole new dimension called the Otherside which is one of the ambient horror type, containing four new environments, more mobs (mostly sculk-based and with base melee AI taken from the zombie) and a structure which has both loot and serves as a way out in case you lose your portal (which is pretty nice).
It features its own biomes (being the deeplands , the echoing forest , the blooming cavern and overcast columns ) with most containing individual blocks and ressources making them worth exploring.
The Transmitter and Staff are very interesting to me, and I would love to see more in terms of sculk-inspired technology, redstone engineering or combat (tool to crawl in walls like how the warden comes out of the ground ? Helmet blinding you but allowing you to witness sounds through walls ? there's so much potential to go wild with these concepts instead of staying satisfied with more powerful variants of already existing minecraft content I'm looking at you Soul Elytra ).

In short, this dimension could be seen as a slightly more interesting mining dimension with a unique horror twist in the form of being the home of everything sculk, while still having some extra content.

To start with the flaws, aesthetics and sounds matter to me, and I don't really like that most mobs just rely on the default player hit sound effects nor emit ambient sounds (the later could still be explained by the dimension being themed around darkness and silence technically speaking). I usually like it better if a dimension has a custom ambience or soundtrack, but to me it isn't an absolute prerequisite when it comes to the Otherside since it is sort of the Warden's lair and an interpretation of where the sculk comes from, so even if having a whole new ost to listen to while there would of course be amazing, it shouldn't the main priority to focus on when thinking about updating this mod I feel;
Still, I think future updates could at least remove the vanilla overworld music from playing there since it obviously doesn't fit with the more horror feel of the dimension.
Another thing is that entering the dimension requires killing a warden, a mob which is explicitely meant to be avoided hence why fighting it seems quite unbalanced and uninteresting in the base game as it is both fast, strong and resilient too (a combination of those three stats being really high is usually something you want to avoid while designing an enemy you are supposed to fight against, I would recommend checking videos about game design principles themed around enemy design since it could also serve the purpose of giving ideas for more unique behavior for the mod's various custom mobs); With this mod, if there is still a want to need fighting a big foe in order to light the portal, there is a need to change the way the warden behaves in combat or make another creature be required instead (same goes for most warden-related drops, which can already be obtained through other means which I find great).
I have also noticed phantoms spawn within the dimension, which while I love the idea of putting phantoms outside of their sleep mechanic like in the End I feel it doesn't really work that much within a heavily cavernous dimension, but maybe there could be creative ways to have them still work, it is just that if they end up spawning naturally they will constantly bonk their heads against ceiling and walls (and this dimension has a lot of that; maybe make them structure-specific where the structure is designed to take them into account ? idk).

In conclusion

Knowing this dimension mod was made before any and all announcements which are related to The Sift , I would like to say that even if it feels gimmicky at times, it could be turned into something far more interesting and is weirdly on point with some of its features.
The mod is well documented (having its own WIP wiki and outside documentation too + advancements, something I tend to forget about) and while it doesn't have a lot of configuration options, it having a config file is still better than nothing at all.
While this mod has plenty of flaws in its current state, I have no doubts it has great chances of changing with later updates as it is clearly stated on the mod's page that there will be more in both content and tweaks as there might be an added internal progression within the dimension;
Do not forget that they have a discord server where you can give feedback to and actively participate in overhauling this dimension mod, which has great potential while not that interesting currently (check out the mod's page, the link is there to be found).

Like I've said before : there's potential for this new dimension, and most beloved minecraft features are developed by multiple people drawing inspiration from several sources, so I am glad to see this project encouraging people to interact with it in this way, discussions are the best way to obtain a mod which might be liked by most.

The ancient city portal is a matter of great discussion in the minecraft community - and this mod seeks to show us what lies beyond it with the Otherside.

This new dimension explores the home of the sculk, and tries to create an atmosphere on a similar level as the Vanilla Nether - dreary, creepy and hostile.

The mod is cool in concept, but is wasted potential. It suffers from illogical progression - To access the portal we need to kill a warden, and said warden is actually stronger than anything in the otherside?
The mod features a powerful boss in its own right, the stalker. The boss can be found in a hidden structure in the Otherside, but can also just randomly spawn from pots in the ancient city? It can be used to make one of two items which are of great importance - an armored elytra.
So there's really only one more item to get from this mod. What's the reward for navigating this desolate sculk wasteland? A transmitter which allows you to access a storage container from anywhere. Pretty lame, in my opinion.

The dimension itself also is just a bit mundane - vast swathes of land without much to do, only one or two interesting natural phenomena - and a variety of building blocks being the main allure. It suffers from a lack of content, and is comparable to pre 1.16 nether.