Does my Galaxy S6 have an unlocked bootloader? I want to install the Resurrection Remix custom ROM on my SM-G920I (Australian Version). Before I start doing anything, I need to make sure that the bootloader is unlocked, so I don't brick my phone. 

To find out the state of my bootloader, I tried doing a few things that I read online. First, having a look at download mode, by holding home, power and volume down on startup. It instead goes to "Odin Mode" and lists these items.

ODIN MODE
PRODUCT NAME: SM-G920I
CURRENT BINARY: Samsung Official
SYSTEM STATUS: Official
REACTIVATION LOCK: OFF
SECURE DOWNLOAD: Enabled
KNOX WARRANTY VOID: 0 (0x0000)
RP SWREV: B:4 K:2 S:2


Do any of these items tell me about the bootloader?

I then tried using the ADB and fastboot method. Fastboot refused to work for me, saying no devices found. I later found out that Samsung devices do not have a fastboot mode at all.

Lastly, I tried just going to Developer Options and enabling "Allow OEM Unlock", but this feature was nowhere to be found on my device.

How am I able to check if my bootloader is unlocked, so I can root and flash a custom ROM on my device?
Most if not all non-US-carrier Samsung devices come with unlocked/unlockable bootloader; those without the "OEM Unlock" switch are simply unlocked by default. Just proceed with flashing TWRP or whatever ops you need to do next.