In a situation in which there is a server with 14 disks (1TB each) that form a DDP with 2 parity disks and 1 preservation capacity disk, theoretically 11TB of total storage would be available. By default, an alert is added with a threshold of 85% occupancy.
Bearing this information in mind, is it necessary to have some additional % available to avoid a performance loss or could 100% of the remaining space be used?
I would add that the alert is primarily used to help track free capacity in the pool when it is configured with thin provisioned volumes. It helps users track when the pool is nearing full capacity and have limited space available to expand the thin volumes automatically / on-demand.
With thick (standard) provisioned volumes, the alert means the pool capacity has been allocated for x number of volumes at the reported percentage. Since the storage is block based and have no insight into the actual usage of standard volumes, the alert is not useful in this scenario. You can disable the alert when the pool is configured with thick (standard) volumes.