I already know that the USB Type-C ports on this machine support the
following alternate mode configurations:
- 4 lanes using Thunderbolt 3.0.
- 4 lanes using DisplayPort 1.2.
- 2 lanes using DisplayPort 1.2 and 2 lanes using USB 3.1.
But I'm pretty sure it supports additional variations.
I'm pretty sure the USB-C ports on Apple computer support only the modes you listed. I'm curious, why you believe there are more alternate modes supported? What do you believe those additional modes might be?
Looking up the supported USB-C alternate modes I can find HDMI, MHL, VirtualLink, audio adapter accessory mode, and debug accessory mode.
MHL, HDMI, and DisplayPort are all video alternate modes for USB-C that came out before 2017. Apple has little motivation to support multiple video modes, especially when they offer such similar features. DisplayPort won out no doubt because it was already integrated with Thunderbolt, getting DisplayPort and Thunderbolt to play nice with each other on the same cable was a solved problem.
MHL, like DisplayPort, can operate on 1, 2, or 4 high speed data lanes. This means it could share the USB-C connector with USB 3.x but this is a protocol not widely used outside of Android devices that used MHL on mini-USB-B ports. MHL seems to have died with the 5 pin mini-USB-B. HDMI takes all 4 high speed data lanes and so it makes sense for Apple to choose to support HDMI on their mini-docks with DisplayPort to HDMI circuitry, this preserves the use of 2 data lanes for USB 3.x.
VirtualLink was introduced in 2018 and is a protocol that is used for 3D goggles. This is a pretty high end feature and so if Apple supported it then I'd expect them to advertise this where we'd notice. I suspect that Apple would expect people that want to use 3D goggles on their Apple computer would just use Thunderbolt rather than VirtualLink. A VirtualLink GPU can certainly be added by Thunderbolt.
The audio adapter accessory mode was added to the USB-C spec in 2017, this is a mode that allows for analog audio over USB-C. If Apple supports this then I would not be surprised. They've been pretty quiet about supporting this mode if they do support it. This is a bit redundant on devices that have a headphone port in addition to USB-C. Apple sells USB-C to headphone jack adapter but this uses a digital connection, proven so by it working fine on computers that offer no analog audio in their USB-C ports.
I'm not sure when the debug accessory mode was added to the USB-C spec. This is a mode for accessing low level data on electronic systems, but then Thunderbolt can do this too. I can't imagine Apple supporting this as it is rather redundant to Thunderbolt and potentially opens the system up to security issues. This is a pretty wide open spec on what the mode does and how it works. It could be allowed as a means put any of a number of existing protocols on a USB-C port, or allow Apple to just make up their own protocol. Again this would be rather redundant for Apple to use given the very capable Thunderbolt protocol, as well as USB and DisplayPort too, that they implemented on USB-C.
USB 3.2 and USB 4 are more recent versions of USB modes for use on USB-C, and allow for all 4 high speed data lines for USB data. Anyone paying attention to Apple's product line knows Apple supports USB4 on their latest hardware. What the USB people like to do is rename old USB standards as parts of the new USB standard so by definition all Apple computers with a USB-C port support USB 3.2. USB 3.2 Gen2x1 is identical to USB 3.1 Gen2, USB 3.2 Gen2x2 is then adding use of all 4 data lanes for USB data and by doing so doubling the bandwidth. USB4 uses all 4 data lanes too but USB4 can intermingle with Thunderbolt and DisplayPort on those data lanes.
That's all a rather long winded way to say that there's no real evidence of Apple supporting any modes on USB-C beyond what you listed. That is until the USB people decided to rename their old protocols again and Apple decided to support USB4. Given how little USB 3.2 and USB4 adds to what Apple already did on USB-C there's not a lot to be excited about. I did speculate on why Apple doesn't support more modes than those you listed, and being speculation from an educated guess I could be proven wrong. If there's more modes supported then Apple has been keeping it well hidden.