The feature is available, but only on (older) models with HDMI+Composite output, and only in Composite output mode (aside from firmware bugs/limitations of older models): they have a display aspect ratio option (4:3 or 16:9).
HDMI-only Roku models only output 720p/1080p/2160p (all widescreen 16:9 resolutions), so this setting is moot for them.
Modern widescreen TVs/Display devices generally handle aspect ratio settings otherwise.