In the case of RDR2 it seems like that's almost certainly a reference to the old-timey film cameras contemporary to the setting. They frequently featured a prominent vignette effect.
Whether a video game camera should ape the limitations of an optical camera in order to produce a "cinematic" effect is an open debate, but, in this particular example, I can see and support the artistic intention behind incorporating the effect.
Whether the intention or effect was crucial enough to make the vignette mandatory is another debate entirely.