I've heard back from Google support, and they've confirmed there are currently no plans to support SVG images in the proxy. They said they account for only 1 in 100,000 email images.
... Meanwhile, everyone is changing SVG images to other formats precisely because they don't support it... Good job Google.
Checked in betalabs
- the image urls in gmail is changed e.g. before the fix: https://ci5.googleusercontent.com/proxy/Oud_2pyP[..]#https://en.wikipedia.beta.wmflabs.org/w/extensions/Flow/modules/notification/icon/flow-new-topic.svg
- the screenshots below show email notifications with icons displayed for the following events:
- Flow pages notifications (e.g. a new topic was created)
- User rights change
- Educational program enrollment
- Revert edits
- Page link
- Page review