What stands out to me is the way “three” & “free” have become indistinguishable among all classes now with the dental fricative being lost. It’s a shame since it’s a unique sound only a few languages including English.
Well, there's a reason why it's rare as a phoneme - it's not only more difficult to articulate than more common sounds, but also more difficult to distinguish when you already have a voiceless stop and a voiceless fricative articulated nearby. Many other Germanic languages lost it for the same reason, and the same process is ongoing in Arabic.
The sound by itself is not quite as rare, usually as an allophone of /t/ or /s/ in some contexts where it's easier to articulate.
