It might be slightly victim blaming but if a cheap-to-apply sticker prevents a few silly cyclists from putting themselves in a dangerous position then it's worth it. And I think more cyclists than you give credit for do see these signs and act more cautiously/assertively even if they still choose to filter on the left. You can almost see the brain ticking with a certain 'type' of cyclist when they see these signs, they consider why such a sign is on the back of the vehicle before deciding to make the move or not.
Yes the majority of cyclists on these forums are probably reasonably safe and sensible but there are plenty of newbie/naive/dumb/downright dangerous and reckless cyclists who will willingly undertake a lorry indicating left 5-10 metres before a junction (ditto filtering on the left of buses which are already indicating left just before a bus stop). If it makes at least a few of them consider why it might be a bad move then it's a good thing imo. It's not excluding the possibility that we should be making efforts to improve visibility in lorry cabs etc.
Yes the majority of cyclists on these forums are probably reasonably safe and sensible but there are plenty of newbie/naive/dumb/downright dangerous and reckless cyclists who will willingly undertake a lorry indicating left 5-10 metres before a junction (ditto filtering on the left of buses which are already indicating left just before a bus stop). If it makes at least a few of them consider why it might be a bad move then it's a good thing imo. It's not excluding the possibility that we should be making efforts to improve visibility in lorry cabs etc.