Are you asking about OVFs in DSLRs? In all the digital cameras I have used, the EVF (if present) and the rear-screen LCD show just the selected crop surrounded by black, typically using the dead-space for extra info display. But some posts suggest that Sony's EVF cameras do not do this.
Sony only provides the standard 3:2 for stills and 16:9 for video. I believe it is the same for Fuji apsc cameras..
Nikon provides a 5:4 aspect ratio in the OVF in their higher end cameras DSLRs (D810, D800, D4 etc etc).
Canon only provides different aspect ratios in "live view" in their DSLRs, which is something but sorta defeats the purpose for me..
My annoyance with Sony is that they provide different aspect ratios in their consumer RX100 series and the fixed lens RX1R2 (3:2, 16:9, 1:1 and 4:3) but do not provide it in the high end A7RII or A9.
I sold my A7RII for the Fuji GFX and this is one of the main reasons. I LOVE the different aspect ratios it provides in the EVF (4:3, 1:1, 7:6, 5:4, 3:2, 16:9 and 65:24)
And the RAW file retains all the sensor data for post cropping.... Awesome!