There is a time limit after which replays are unviewable. The current cutoff is somewhere around 2019.
The game client from 2014-2017 is too different from the current one to allow viewing replays from that time period. Even the maps look different and have different mechanisms like Tower Ammo that no longer exist.
There are also abilities like Zeratul’s Shadow Assault heroic that are completely deleted from the game files, making viewing impossible even after downloading an old client.
If you try to view replays from a year like 2017, 2016 etc. the client will crash every time.