If you look at the assets folder in the www directory you will see a folder called fonts. It contains the icons plus the font noto-sans (which I think is for IOS) and roboto (for Android).
This adds over 2 Mbytes to the final package size - does anyone know what the fonts are used for?
So the fonts are / can be used as a default option but as you properly pointed out they might be an excessive asset as well. In a lot of articles about optimization you can see people are talking about getting rid / tuning this part: https://julienrenaux.fr/2017/07/20/optimized-ionic-angular-css-bundle-for-pwas/
i have removed the fonts and see no difference - but I assume there must be some reason why Ionic puts them in originally. There does not seem any sense including the font the android when using IOS and vice visa.