Some Lenovo laptops will not boot non-Windows OSes (rectified)
These are machines running "Windows 10 Signature Edition". It is implied that the locking is a 'feature' of the "Signature Edition" specifically, implemented as a contractual requirement between Microsoft and the OEM.
There are reports that the "Signature Edition" nature of some models of laptop are not clear when ordering.
It appears that this restriction is implemented not via code signing practices, but by forcing the BIOS to configure the storage controller in RAID rather than AHCI mode, a mode which the Linux kernel does not (at this time) support. However, this would normally be configurable via BIOS menu; the option has been removed. It appears that this was deliberately done to inhibit the use of other operating systems.
As such, this is not strictly an OSIGNOS violation, but the underlying intent is the same as that of OSIGNOS.
This was rectified some time later when Lenovo released a firmware update to allow the setting to be changed.