Hi Séverin,
Some of these things are doable with a work-around using some simple custom CSS to hide the buttons. For example
body.path-mod-forum button.atto_emojipicker_button {
display: none;
}will hide the emoji button in all forums.
Certain plugins are already somewhat context sensitive. The equation editor plugin requires the MathJax or TeX notation filter to be enabled otherwise the button is hidden. I have implemented context sensitivity in atto_cloze and atto_fullscreen plugins which I maintain. The cloze button only appears which editing a cloze question, and full screen can be enabled only for editing teachers. One might request other maintainer to implement similar features where it makes sense.