Yes Stem Cells can be frozen and when you have frozen stem cells the frozen stem cells and cord blood is expected to last basically forever.
However cord blood banking and stem cell storage has been in existence for 25 years as of 2014 which is how long some stem cells and cord blood has been banked and have been found to remain viable.
Actual stem cell and cord blood banking though now as of 2020 has been in existence for 31 years and so far we don't know if any stem cells or cord blood will remain viable for the 30 years or so.
Stem cells and cord blood banking has been around for at least 25 years and so far stem cells and cord blood has remained viable for those 25 years so far.
So we do know that stem cells and cord blood remain viable for at least 25 years when it's stored properly using cord blood banking.
However since cord blood and stem cell banking have not been around longer than 25 years as of now we cannot know how much longer stem cells and cord blood remain viable.
Hopefully the stem cells and cord blood never need to be used but it's likely that the stem cells and cord blood will remain viable for 30 to 50 years or even longer.
But most people would not be around or want to store cord blood or stem cells longer than 50 or so years anyway.
But at least as of now we know that the cord blood and stem cells are still viable and usable after 25 years of storage.