Update 2020-05-04: We've finally published Core and TKLDev as stable releases (v16.0); along with a few others. The RC releases noted here are no longer available for download. Please see the first instalment of updated appliances. Update 2020-05-15: Please see the second instalment of updated appliances.
UPDATE: Stage 1 of v15.0 stable release is now available. It includes 47 updated appliance ISOs. Stage 2 includes OVA/VM, OpenStack and Xen. Docker and Proxmox/LXC builds published too. Stage 3 includes 35 additional appliances; including info on 3 new v15.0 appliances, plus other notes of interest.