I am so not looking forward to Apple's removal of the headphone jack from the iPhone this year. For me Bluetooth is still lousy every time I have to use it.
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It is not a given Apple wireless headphones will use Bluetooth, or may not use the current 'standard' Bluetooth. Proprietary tech is within the realm of possible, Especially when the industry standard is not up to par.
This is true and something that had occurred to me. But that doesn't really help me plug in my Bose noise cancelling headphones, unless they also adapt the Apple alternative (and even then I'd have to spend another £300).
Even if they did, it is still significantly worse than wired. The Bluetooth version of the Bose for example half half the battery life and when you find yourself with the battery dead, they are completely useless when you can't fall back to wired. And of course having to use wireless also reduces the battery life of the phone.
I have not been keeping notes, have the various Apple brand wireless trackpads, mice and keyboards been using standard Bluetooth or special sauce radio tech?
They use standard Bluetooth.
Apple also uses Bluetooth for wireless tethering to iPhone, which for me has been fairly reliable.
I've always used their default of wifi tethering rather than Bluetooth.
Apple Watch uses Bluetooth to link with iPhone. Seems to work well enough?
I own an Apple Watch, the connection back to the iPhone is often the weakest part of the user experience.
In the newest Mac OS they are now sensing Apple Watch proximity to the Mac to provide automatic Mac unlocking, which suggests they think they have a handle on getting it to work reliably.
Let's hope it is more reliable than the existing continuity features

Perhaps Apple hardware on both sides of a Bluetooth link can be more robust than using mixed brand Bluetooth?
Maybe, but even then having wireless be the only option is a big backward step in my opinion.