https://bugs.webkit.org/show_bug.cgi?id=178109 deals with adding arbitrary resolutions to getUserMedia.
From reading that report, it sounds like it might involve some complexity. As a simple first step, I'd suggest adding the QQVGA resolution (160×120) to the list of supported resolutions.
Here are some reasons why I think this would be a good thing to have ASAP:
1. It's a standard resolution (see https://en.wikipedia.org/wiki/Graphics_display_resolution#QQVGA_.28160.C3.97120.29), which I'm pretty confident would be supported natively by most webcams.
2. Ideal resolution for 'thumbnail' videos as are often used in multiparty videoconferencing solutions.
3. Useful as a fallback resolution for users with poor internet speed (as it's 1/4 the size of QVGA).
*** Bug 179362 has been marked as a duplicate of this bug. ***
It appears this has been implemented as of iOS 12.2 Beta, however, there is a critical bug with the functionality as detailed here: https://bugs.webkit.org/show_bug.cgi?id=195868