While Firefox has had non-broken support for SVG favicons since 2015, Chrome only added support last year. There are certainly features Safari is behind on but this isn't exactly a shining example of it.

IE6 was borderline abandonware with hideously broken implementations of many core features. Implying that Safari is comparable to IE6 is just factually wrong. No, it doesn't live on the bleeding edge like Firefox and Chrome, but it has excellent support for non-bleeding edge features. And where there are notable gaps in feature support (e.g. webp images) there are clean, formal mechanisms for fallback.

Usually when people complain about Safari, it's because they care about some bleeding edge feature (which they probably don't need anyway) or one of a few specific absent features such as web notifications. (For which I'll controversially retort with good! It's an anti-feature. Pleased that Safari is holding the line on that one.)