What altimg attribute? Is this part of MathML3 spec?
You might want to see this link:
https://github.com/w3c/epub-specs/issues/1268
The only mention I could find was in the epub 3.1 spec back in 2017
https://www.w3.org/submissions/2017/...docs-20170125/
But since those fallbacks were never supported by any e-readers, support was dropped in later epub3.2 and now 3.3 specs.
So technically these are unused images under the latest spec.