Supporter rank gives:
Access to /fly
Access to /nickname
Access to /kits (supporter weekly (250 Stardust) and supporter monthly (5 Untradable Shiny Pokemon))
/enderchest
/anvil
/sit
/glow

Check pinned messages in the Discord general chat to see how to apply a nickname.

These benefits do go away if you cancel Supporter; please look at your payment receipt for cancellation info, if you need help reach out.