I am a teacher so, sorry, I just like to give tips about stuff where I had trouble. Just box selecting all of the keys/points (in Blender 3.x at least) won't let you see the menu where you find the modifiers. You will just see some menu about showing the cursor. You need to actually manually select a key and then do an L (link select) or an A (select all) if you want all of the frames and the N menu now shows the tabs, one of which will be the modifiers. Note: when you select the key, notice that the curve changes to bold also.
Really. Just trying to help out.
Thanks DDaemon !
These tips of you are really helpful!
But if you simply select a Key, the whole Curve already becomes the Active one (bold), so you don't need to use L (unless you want all Keys of that selected of course...) to see the Modifiers Tab.