In many audio call applications, every user can decide to selectively mute just one of the other users.
For example in a call with A, B and C, one participant (A) could decide to mute another (B) while B is still heard by C.
Does asterisk allow something like this in conference/meetme or other applications? I couldn't find suitable commands from the documentation
You can't do that. No any way mute USER
You can mute channel( all not hear that channel), not user.
You can organize call via chan_spy(one way sound).
You can transfer users from conference into other conference where some channels muted.
Scenario you want is possible, but dialplan to support it will be REALY complex. No way do it by simple tricks.
For example you can have each user into dedicated room and create channels/connection between rooms as you wish, room B connected to C and not connected to A. But again, that will be high cpu usage and really complex.
At current moment both app_conference and app_metmee have ONE mixing function which send packets to all users.