Are dash cams / bicycle cams not ubiquitous in Europe by now?
I would have thought they would be very useful for adjudicating high cost events such as automobile collisions, or even police interactions.
Like most things with "Europe" - it depends, because it isn't one state with universal laws or even cultural expectations. Go to somewhere like Poland and everyone will have a dashcam and it's almost expected at this point(imho). But in neighbouring Germany you are technically allowed to have a dashcam, but any recording is legally inadmissable in any court case. So even if you have a recording of someone crashing into you, it can't be used because the other person never agreed to be recorded. Meanwhile Austria and Portugal have banned them completely, even for personal use.
Germany specifically doesn't like dashcams that record continuously. Legal ones just keep a short buffer and when they detect a crash or a button is pressed (or a voice command in fancy ones) they'll write it to storage. Because if something happens you have a valid reason to have and use footage, you just can't record people without a reason.