I always read that the manufacturer gives it a one year life after opening while being kept in the freezer.
That has to do more with them covering their butts.
I buy yeast in bulk and use it over time. I keep my deep freezer at -8F. My yeast lasts more than 3 years.
Basically, I keep the yeast in an airtight container in the freezer. I keep a small amount always in my fridge. When my fridge supply runs out, I refill from the freezer.
I have never had an issue with not enough living yeast cells.
But given that some people aren't going to keep the freezer cold enough and aren't going to use a completely airtight container, it is much safer for them to say it only lasts a year in the freezer.