I started this explain command about 1 day ago. It is still showing as "Running". A number of queries that ran after this one are still shown at Running or Queued.
About a dozen or more queries that happened just after this query are still marked as "Running". I believe there are others as well, some more queries that followed these.
I think somehow running Explain caused all queries that followed to pile up and never actually be run. But without actually getting killed.
See https://quarry.wmflabs.org/query/15607 for an example
Explain is almost certainly broken by T264254: Prepare Quarry for multiinstance wiki replicas
The functionality will need rewriting or similar. It stopped working reliably long before that though because of load balancing on the replicas.