Target Disk Mode between old MBP and new MBP?

Target Disk Mode still exists – but Apple put a restriction on it that limits its utility.

If either of the computers is running Big Sur or later (which will be the case for all Apple-Silicon-based Macs), the connection must be a Thunderbolt one. That could greatly increase the cost of making a supported connection – or even, in the case of a really old Mac like your 'unibody' MBP, make it impossible to make a connection at all.

Transfer files between two Mac computers using target disk mode - Apple Support

If you have a new Apple Silicon Mac, and you want to put it into a mode where it acts like an external disk, I believe that you could use either USB or Thunderbolt. But having the older Mac act as the "computer" and a new one act as the "disk" is the reverse of what most people are looking for.

Transfer files between a Mac with Apple silicon and another Mac - Apple Support

Your best bet is probably to make a backup of your 'unibody' MacBook Pro onto an external drive that has a USB interface. This could be a Time Machine backup, or a bootable clone backup made with Carbon Copy Cloner or SuperDuper!

Then, when migrating to the new machine, transfer the backup drive to the new machine, and use it as the source for Migration Assistant or for a manual migration.

You will need to compensate for the USB-A vs. USB-C difference, but that's not too hard. When you're talking USB protocol at USB 3 speeds, or less, it is not hard to find adapters and cables to go between any of the common USB drive connectors (USB-A, USB 3.0 Micro B, USB-C).

Time Machine to a USB storage device is a common choice for migration, if that’s where this is headed.

If you’re just looking to use TDM for external storage, it’s workable, but an awfully inefficient and complex external storage device.

Another option for sharing files and storage among computers is configuring the SMB file share server, though how well that works can depend on which OS X or macOS version is in use on the Mac being used as a file server.

It’s been a long while since I’ve tried this with something as far back as OS X 10.11, though.

chocfruit wrote:

What I’m not sure about is how recent any of them are relative to what is on the MBP itself, so I wanted to see if it would be possible to get the data off the MBP directly.

You could try removing the drive from that MBP and putting it into a USB enclosure, or attaching it to a USB-to-SATA adapter.

No guarantee that you will be able to read anything off the drive – especially if "the last time the 2008 machine was switched on it wasn't working properly". Drive failure is one of the most common causes of death when it comes to old computers. But you can get a tool-free USB notebook drive enclosure for maybe $8 - $15.