I think you're conflating different things here. Also you haven't told us what camera you're using, or thinking about, which makes it hard to get specific.
> why there is no 2.35:1 or any other widescreen aspect ratio overlays when shooting a video?
On the R5C, and I guess all the Canon cine cameras, there is. See manual page 96 -- you can overlay aspect ratio markers for any aspect ratio you want. For cameras with the photo operating system, like the R5, you may be out of luck. Of course many external monitors can add aspect overlays.
> Only the standard 16:9 and who knows what else they have that nobody uses.
Are you claiming that nobody uses 16:9? I think a few people may disagree. Me, for one.
> And no, I will not crop my footage in post,
This is baffling. Aspect overlays just guide your framing; they don't change the fact that you need to crop in post. Same as putting tape on the monitor. And why would that be a problem? If you're doing any kind of editing, cropping is simple -- just make your editing movie the aspect ratio you want, and position your footage in the frame wherever you like.
Also, I think you're missing a real benefit here. If you shoot "taller" than your final movie -- say you shoot 16:9, for a movie that will be 2.35:1 -- then in post, you can move the footage up and down within the final frame. One project I'm editing right now does exactly this. Because it was shot 16:9, so I have more height than I need, I can move the rushes up and down to align the characters' eyelines -- super useful.
> because I'll loose a lot of information from my footage, I simply want to frame my shot from the beginning to make it look proper in widescreen aspect ratio.
I don't know how you think this works, but I suspect you need to rethink. You seem to be saying that if the camera read out a 2.35:1 ratio directly from the sensor, you would not "loose a lot of information from my footage". But, uh, yes, you would, because the camera would only be reading part of the sensor. This would produce EXACTLY the same result as cropping in post.
Or maybe you're saying that the sensor itself should physically be wider -- like instead of 4096 × 2160, like the C70, it could be 4096 × 1750. Which is true, but this is fewer pixels -- which again would achieve EXACTLY the same resolution as cropping in post. Maybe you would say OK, go to a more dense sensor, like 8192 × 3500. Which you can do, in fact, by using the R5 or R5C. The fact that there are extra pixels on the top and bottom -- because the height is actually 4320 -- doesn't mean that you're losing information.
Bottom line, since the image circle produced by the lens is a circle, taking pixels off the sensor to make it a wider aspect ratio would really be throwing away potential information.
Then again, as Ricky pointed out, if you really want to go wide, shoot anamorphic.
