I frozen a lot of yeast over the years and never needed to use any of it, so I don't even know if it is viable.
All the strains I have frozen, I have managed to keep viable by other means, and all are available to me today.
When I first use a yeast, I make a starter, I make it a quart bigger than I need. I then take the extra quart, drain the liquid, and store the yeast with distilled water in an 8 ozs mason jar in the fridge. I have found this saved yeast can be revived after 24 months (although I try and use it within 12).
When i use a yeast, I generally run a few beers with the same yeast, so I take some of the slurry from one beer and use it for the next one.
I have a couple of yeasts that I have used for 20 batches, and still have a fresh sample in the fridge.