Have you just checked this ? Can you prove it ? Have you ever tried streaming slides let's say at 1080p60 @ 8Mbits to see what happens ? Or are you just assuming ?

Frame rate has no impact at all on bandwidth.
Only bitrate and frame complexity can affect bandwidth use.
A 10fps video stream @ 5Mbits will consume exactly the same bandwidth as a 30fps video stream @ 5Mbits. By "video stream" I mean ever changing images as opposed to static content. However with static content, neither the 30fps nor the 10fps streams will use 5Mbits defined bitrate (hence the bandwidth) because of the nature of the content which encoding is much more simple and will result with an as effective image quality with a lot less bandwidth.

By default vMix, set for let's say 1080p25 @ 8Mbits streaming (yes there is a PAL world too, not only NTSC), will generate a 800Kbits stream when only static slides are used. It's due to how H.264 works, it has nothing to do with framerates.

Besides vMix is professional video production tool, it conforms to professional video standards.

I note that inversely to what you intitially said (about "unnecessary bandwidth at the high resolutions being used"), you're now justifying your request by "bandwidth spikes that come during things like crossfade transitions" - which btw will look like shit at 10fps - that's a totally different thing... Framerates were initially determined to provide acceptable smoothness for motion and transition effects in video productions.

Sorry for making you feel like an ass, that was not my intention, I just wanted to make you realize that your statements were not true, but it seems you do not really understand how interframe codecs work and you're confusing with "intraframe" or intermediate codecs.

Sorry again but I think your feature request is not justified in the context of vMix being a live video production software respectful of the standards. Moreover you can use OBS or FFMPEG behind vMix if you want to push non standard framerate streams to the internet.