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.

I wonder if you could (ab)use ResourceLoaderImageModule, and the load.php?image=…&format=rasterized URLs it generates, for a fairly easy workaround?

Change 304299 had a related patch set uploaded (by Mooeypoo):
Add a rasterized email icons to Echo modules for email

https://gerrit.wikimedia.org/r/304299

Change 304299 merged by jenkins-bot:
Add a rasterized email icons to Echo modules for email

https://gerrit.wikimedia.org/r/304299

Checked in betalabs

1. 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

after the fix
https://ci6.googleusercontent.com/proxy/qerRsnzR[...]#https://en.wikipedia.beta.wmflabs.org/w/load.php?modules=ext.echo.emailicons&image=flow-new-topic&lang=en&format=rasterized

2. the screenshots below show email notifications with icons displayed for the following events:
   • Flow pages notifications (e.g. a new topic was created)

• Thanks

• Mention

• User rights change

• Educational program enrollment

• Revert edits

• Page link

• Page review