ant deployment fails with "Error: limit exceeded"

Ended up making a case about this issue. Support ran a Physical Delete (PD) on my org to delete custom fields that have been deleted but still remain. Deployments are working now.

I've also asked about the purgeOnDelete flag when running an ant undeploy job. Here is their response:

"That's slightly different. Essentially, the OnDelete flag puts the field in a 'hard' deleted state. We then have a sweeper program that will remove these later ('physical delete'). This is random and usually occurs nightly. In situations like ours here, we can manually run the physical delete, instead of waiting for it to occur organically."

In our case I checked Setup > Background Jobs 

There was a Job Type there called "Cleanup of custom field data when a custom field definition is hard deleted"

The Status showed "Completed" so I retried my deployment validation and this time it completed successfully.