I noticed these messages from dmesg after a reboot.
[ 10.557939] thinkpad_ec: loading out-of-tree module taints kernel.
[ 10.558304] thinkpad_ec: thinkpad_ec_request_row: arg0 rejected: (0x01:0x00)->0x00
[ 10.558305] thinkpad_ec: thinkpad_ec_read_row: failed requesting row: (0x01:0x00)->0xfffffffb
[ 10.558307] thinkpad_ec: initial ec test failed
[ 10.639393] thinkpad_ec: thinkpad_ec_request_row: arg0 rejected: (0x01:0x00)->0x00
[ 10.639394] thinkpad_ec: thinkpad_ec_read_row: failed requesting row: (0x01:0x00)->0xfffffffb
[ 10.639397] thinkpad_ec: initial ec test failed
Googling I found this thread from https://github.com/evgeni/tp_smapi/issues/14 that started in 2012 and continued to 2015.
The gist seems to be that I should remove tp-smapi-dkms because those features are not supported by the hardware.
apt-get purge tp-smapi-dkms
And on reboot the messages have gone away:
dmesg | grep think
[ 2.661884] thinkpad_acpi: ThinkPad ACPI Extras v0.25
[ 2.661885] thinkpad_acpi: http://ibm-acpi.sf.net/
[ 2.661886] thinkpad_acpi: ThinkPad BIOS N1GET70W (1.49 ), EC unknown
[ 2.661886] thinkpad_acpi: Lenovo ThinkPad Yoga 260, model 20FDCTO1WW
[ 2.671689] thinkpad_acpi: radio switch found; radios are enabled
[ 2.671850] thinkpad_acpi: This ThinkPad has standard ACPI backlight brightness control, supported by the ACPI video driver
[ 2.671850] thinkpad_acpi: Disabling thinkpad-acpi brightness events by default...
[ 2.675346] thinkpad_acpi: rfkill switch tpacpi_bluetooth_sw: radio is unblocked
[ 2.678097] thinkpad_acpi: Standard ACPI backlight interface available, not loading native one
[ 2.681189] input: ThinkPad Extra Buttons as /devices/platform/thinkpad_acpi/input/input7