@Chiara Sirna 

I wish people could be able to write on the table without neither modifying its structure nor deleting others’ contributions

Tables used to be called Grids and allowed for editing of the cells when the table was locked - this is no longer the case.

Unfortunately, you can also not lock objects when they are placed on top of a frame, so asking you participants to enter their comments in sticky notes, shapes, text, or comments* won’t meet your requirements as the participants won’t be able to lock the objects - and even if they could, the Free Plan team doesn’t not support protected lock which blocks any other board members from deleting objects.

I would suggest that you consider using a series of locked frames or shapes and asking your participants to enter their comments into sticky notes, placing them on your frames or shapes, and locking them. That is about the closest you will come to avoiding people modifying/deleting others content.

*Any board Editor can delete any comment on the board. So, now that I think about this, the only way you could have team members on a Free Plan not be able to edit any board context, but modify it in any way, would be to set the board’s Share settings to Team access to board → Can comment.