Looking at it I would say it is a torsional crack with a hollow inside. Sounding the trunk with a mallet would be quick and an easy way of knowing if it's hollow. If you really want to keep the tree a extensive crown redux would be needed and possible some bracing. But that depends on the cause of the decay and the extent of it. As always a little hard to do diagnostics from my recliner.
1/4" diameter rod to probe and a mallet are your best tools to revisit the tree. Put away the pruning saw until you know more about what is going on there. There may be little strength loss...or may be significant loss.