It's always been risky to trust the mute button, even before zoom etc. The rules have been the same for at least 30 years. Never send an email or other type of text message you don't want the whole world to see. Never say anything on a conference call, even when muted, you wouldn't say when not muted.
Ethically any audio chat software shouldn't transmit any audio it receives when muted. The "hey, you are muted" notifications can all be done client side and don't need any server side support. But ethics is not a factor in the design of any enterprise office software.
It's been mentioned a few times in this thread, but it looks like they all have a legitimate reason for this which is making sure the unmute button is instant. Because having the first few words of your talking get through is more important than a theoretical privacy issue to most users.
Ethically any audio chat software shouldn't transmit any audio it receives when muted. The "hey, you are muted" notifications can all be done client side and don't need any server side support. But ethics is not a factor in the design of any enterprise office software.