From this New York Times article:

Apple said it was planning an iPhone software update that would effectively disable the phone’s charging and data port — the opening where users plug in headphones, power cables and adapters — an hour after the phone is locked. While a phone can still be charged, a person would first need to enter the phone’s password to transfer data to or from the device using the port.

And from the Elcomsoft blog:

In the second beta of 11.4.1 released just days ago, activating the SOS mode enables USB restrictions, too. This feature was not present in the first 11.4.1 beta (and it is not part of any other version of iOS including iOS 12 beta). In all other versions of iOS, the SOS mode just disables Touch/Face ID. The SOS feature in iOS 11.4.1 beta 2 makes your iPhone behave exactly like if you did not unlock it for more than an hour, effectively blocking all USB communications until you unlock the device (with a passcode, as Touch ID/Face ID would be also disabled).

And this from Motherboard, with the title Cops Are Confident iPhone Hackers Have Found a Workaround to Apple’s New Security Feature:

“Grayshift has gone to great lengths to future proof their technology and stated that they have already defeated this security feature in the beta build. Additionally, the GrayKey has built in future capabilities that will begin to be leveraged as time goes on,” a June email from a forensic expert who planned to meet with Grayshift, and seen by Motherboard, reads, although it is unclear from the email itself how much of this may be marketing bluff.

And:

A second person, responding to the first email, said that Grayshift addressed USB Restricted Mode in a webinar several weeks ago.

My instinct is that this is, indeed, a marketing bluff. But one without teeth if it doesn’t work.

Whac-a-mole (note the spelling, a trademark thing, I think).