I mute my microphone (its input volume is at 100%, just muted). But it always stays muted until I unmute. Even between logins and reboots. I do use pipewire, so there’s a difference. But I believe pulseaudio did the same for me back in the day.
One solution could be to mute the microphone in your DE or WM initialization file. Which are you using? Gnome? i3 perhaps? You could use pactl to mute the microphone on each login.