The trouble is you are assuming the dictionary definition of ‘mandatory’ applies, really they mean ‘do this or there will be consequences’ .When looking at the dive on my laptop at the safety stop one of the caution triangles says mandatory.
Glenn
The consequences, as far as the computer is concerned, are minor ones concerning NDL adjustments. Of course there may be physical consequences too so it is generally best to follow the instructions.
In BSAC the standard 35m training involves a simulated deco dive. Given half a chance, but only once qualified to an appropriate depth, I try to have students get the computer into deco in controlled circumstances so they have an idea what to expect to see. It turns out to be quite fiddly to arrange, ideally take two computers, dive Nitrox and set one computer to air.
Going out of you way to understand what might happen is better, in my opinion, than when accidentally overstaying at depth being confused about why the NDL numbers are going up and arriving at stop depth still confused and short on gas for a real deco stop.