Just to clarify, plants don’t have, and never have had, B12. B12 is a byproduct of bacteria action, and yes, it used to be on our foods that came out of the ground – assuming you didn’t wash it well.
Yeast, even nutritional yeast, does not have B12 unless it has been fortified. Make sure you are consuming fortified (often labelled as “vegetarian support formula”) yeast.
Humans can also store years worth of B12, so it is very possible to not be consuming enough and not have any problems… until you do.