The left crankarm of an HollowTech II crank has a little part called "safety plate" (number 4 on the drawing below). What is exactly its role, and what kind of 'safety' does it offer given the forces applied on a crank, it's unlikely that it will make a difference if the bolts get loose)? Is it a problem if it is missing?
Based on my observations from many Hollowtech II installs, I think it serves to retain the left crankarm (and thereby the whole crankset assembly) together if the two left crankarm bolts loosen (but have not backed out completely). It is made of a relatively hard enough plastic to provide some retention, and the side loads on a crank are not the primary loads. Eventually the plastic would fail, but hopefully an aware rider would notice that the precision of their crankset is degrading (clunking, loose) and would stop to investigate (in the same manner that if they detected that their front wheel was loose).
The basis for this is that both of the left crankarm bolts are engaged with the safety plate. The inner bolt completely retains the plate (it passes through a hole in the plate), and the outer bolt positively retains the plate when the plate is engaged into the Hollowtech II axle, as the hard plastic plate deforms slightly to "snap" over the outer bolt. In this installed position, the plastic tab that inserts into the designated hole in the Hollowtech II axle prevents the left crankarm from sliding off the axle if the two retention bolts were to loosen. The plastic is hard enough to retain the crankarm unless an unreasonable force is applied.
Its function is similar to the retention tabs (a.k.a., lawyer nubs) present on a front fork with a quick-release axle. The retention tabs prevent the front wheel from falling out of the front fork if the QR is loosened. Similarly, the crankarm safety plate prevents the left crankarm from falling off the axle if the two retention bolts loosen.
What about the cap (not shown in the diagram above) that "preloads" the assembly? The cap will also retain the crankarm on the axle. With properly torqued crankarm bolts, the cap is not going to move as the clamping force of the crankarm increases the torque required to loosen the cap enough that it should not move. However, if the crankarm bolts loosen, the cap becomes considerably easier to loosen (installation torque is roughly 1.0 N-m), and theoretically could then loosen. At that point the safety plate may be all that is retaining your crankarm. Safety defense-in-depth here. They call it a safety plate for a reason.
It tries to prevent incorrect assembly.
The axle has an indent to accommodate the pin of the safety plate. If you do not push the crank arm all the way onto the axle, the safety plate will not sit flush and will not allow one of the crank bolts to be inserted.
It has no function once the crank arm is mounted. When mounting a crank arm without the safety plate you need some other means (caliper) to make sure that there is an adequate length of engagement between the axle and the arm.
The Park Tool site on step 3a for "Crank Removal and Installation: Two-Piece Compression Slotted" says:
For Shimano® Hollowtech® II, inspect for a “stop plate” inside the left arm slot. Use a thin screwdriver or integrated hook on the BBT-10.2 to lift this plate upward. The stop plate acts as a safety redundancy to prevent left arm removal. FSA® has no stop plate.
My thought would be that if you tighten the bolts correctly and check them periodically, you are fine.
I'd want to replace the stop plate at some point just to satisfy my OCD.
