You could very well be correct. These are older line cards, so that could actually be the reason for the documentation lagging the feature. Which is fine, as long as that is the case. I'll buy new cards. But i would need to know which cards would support this. I can have a discussion with my sales guys, though.
I think my biggest issue is... If the MX doesn't support this on these cards, then that means there was a time when there was literally no way to perform layer 2 security on the MX. Bridge domains have been supported since version 9.x, maybe even before.
I'll try to figure out which cards do support this feature.