Sidetone in a headset is basically when you can hear your mic audio through your own headphones speakers. I absolutely hate this. I consider this to be an anti-feature. In a normal pair of headphones, I could see where this would be bearable. But I greatly prefer noise-canceling headphones. Sidetone plus noise canceling means you’re hearing exclusively the garbled mic noise, since all of the actual background noise is canceled out (but then re-introduced through the sidetone).
The bose QC-35 II is generally my favorite pair of headphones that I currently own, but it has strong sidetone that cannot be turned off, and I hate that.
I recently got the Jabra Elite 85h, and it’s companion app has a setting to turn off sidetone. But the problem with the 85h is that the sound quality is absolutely garbage (in both bluetooth stereo (for music) and mono headset modes). And also, the noise cancelling is not as good as the bose.
Tangential complaint: both of these bluetooth headsets behave erratically when you try to switch from one paired device to another. So, for instance, switching from phone for music-listening to work laptop for a conference call is difficult and error-prone.