I tried watching an episode of The Simpsons yesterday. It’s a direct, lossless remux of an NTSC DVD, in mkv format, MPEG2, 480i/60, 4:3 aspect ratio; output resolution set to 480p; hardware acceleration on.
The “View Mode” setting seems to have stopped working entirely: every value for that setting looks exactly the same. (And every setting is now doing the thing of horizontally downscaling the image - usually one can avoid that by setting View Mode to "Stretch 16:9”).
Disabling hardware acceleration seems to work around the problem.
576 and 480 aspect ratios are a work in progress atm. With hw acceleration on, we are bypassing all Kodi’s scaling so 16:9 material should be as good as it gets (interlacing notwithstanding). But I’m still ‘postboxing’ 4:3. Next step is to pass the 4:3 aspect ratio metadata through the kernel drivers to achieve what you have been asking for for a while now.
Having some trouble reliably identifying a 4:3 aspect ratio though. @hissingshark has pointed out some funnies on broadcast TV from COM8, eg Forces TV.
That’ll be nice, certainly. 
If it were up to me, in the interim, I would rather have everything play as 16:9 and change the aspect ratio with the TV remote rather than losing horizontal resolution on 4:3 stuff. But I can decode in software and use the central “Stretch 16:9” setting for now. (That’s not ideal as it can mess up 4:3 stuff at above SD resolution).
I’m thinking we need an extra option on the 4:3 menu: ‘pixel-for-pixel’ or something - to handle passthrough of 544-wide streams. But there’s no HDMI VIC for 544 wide.
In theory we could support custom resolutions and have a VIC for custom modes. Would need to thoroughly ensure a reset each time though.
There are any number of situations in which that might be useful. Native output of 720p/24 and 480p/24 would be handy, for example.
(Or did you just mean resolutions and not refresh rates?)
This would be for resolutions; like 1280x768.