Doom Eternal is created by what is now a first party studio for Xbox. There are no excuses in this case. They will have access to all the resources they would ever need.
At this point you're just guessing to make excuses. We can only talk about the facts that are presented to us today and they state there is no ray tracing for the Series S on Doom Eternal. If it comes at a later date then the question needs to be asked as to why it took more effort or time when people were saying it's a machine that won't have any impact or cost developers time because all they have to do is "move some sliders down".
You can't just ignore all the games where it doesn't perform equally in terms of raytracing (or framerate since the goalposts have now been moved to interpret the marketing as "framerate") and then say "but look this handful of games it does do what they said it should". The results so far have been wildly inconsistent and of you are are Series S owner you cannot be sure of whether or not you will be getting ray tracing in future titles.
It wasn't marketed as a "sometimes" or a "maybe" machine, the statements being made were pretty definitive.
1: it is a first party studio... for like a few months now. and one that uses their own Engine, which doesn't magically get 100% optimized for Series S the moment they finally sealed the deal.
2: I am not making excuses, I think rationally about this case. the reason there is no RT on Series S could have many reasons, one of which can be that they didn't have time to work on, let's be honest, the least sold and therefore least critical version for them. (this patch is most likely in the works for many many months already)
another reason could be that their RAM management would need to be adjusted, the BVH structure might need too much ram, or sucks up too much bandwidth. this means the RT mode on Series S would need additional adjustments which would need more time than the targeted patch release, and they didn't want to push back all the other patches just so they can sort it out.
and games where it doesn't perform as well are rare. and often it's due to weird decisions by developers. See Resident Evil 8... why the fuck is the RT mode running at 1440p? why not drop it to 1080p? same question also for Series X and PS5 tho, why not have an additional RT Performance mode that runs at something like 1800p60?
you will find that developers often do weird shit that doesn't make sense (sometimes it makes sense once you know more details, but not always) you can't simply take a game out of many, point at it and then suggest the hardware is at fault, especially when we know from the past that consoles that aren't the priority can get shafted by developers with way less optimization work being done on them.
or how else would you for example explain how, say, Ace Combat 7 runs at the same resolution and settings on PS4 Pro and One X, but performs better on Pro? The One X is literally better than the Pro in every way, there's nothing the Pro is better at, in fact many parts of the hardware are way worse than in the One X counterparts.
fucking Minecraft runs better on Switch than on One X (I know, I played and have it on both systems) it loads faster and runs smoother... why is that?
there are many reasons why specific games might perform worse than you would expect on some platforms. neglect, issues with APIs, time pressure + too many systems to work on or engine stuff that affects a specific system due to that system's hardware setup
so the 2 possible reasons why Doom Eternal doesn't have an RT mode on Series S that I see are
A: the team simply didn't have time to polish up all the 4 different systems they are targeting
B: the Series S's RAM size or bandwidth clashes with how the engine builds the BVH structure and adjustments need to be made, which takes time (which is also kinda reason A but more elaborately explained)
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