This is becoming more visible with @Krinkle 's efforts to reduce CSS embed usage: T121730: Audit use of @embed and remove where not needed (2019)
SVGs that used to be embedded in gzipped CSS files are now becoming separate requests, where they aren't compressed anymore, despite being textual content:
content-length: 221 content-type: image/svg+xml
(and no content-encoding header to be found)
XML's repetitive nature means that we should always gain from compressing SVG files.