@awesome_guy does keyboard has any logo on the left side of Space key? If no then keyboard is not backlit.
Does keyboard backlit works in UEFI? It should.
Also remove your keyboard using Hardware Maintenance Manual for L440 and look for backlit cable, it could be damaged or poorly connected.
Shop owner is saying that the key was probably replaced and has backlit logo on it and the laptop doesn't have backlit. I am not entirely sure that his statement is right or wrong. Could it be the case that some L440 have backlit and some doesn't i.e. 2 variants of this tinkpad?
https://psref.lenovo.com/syspool/Sys/PDF/ThinkPad/ThinkPad_L440/ThinkPad_L440_Spec.PDF
PSREF implies that no L440 should have a backlit keyboard. Which makes sense the L series are the cheap models so I'd expect them to not have it. Especially since the *40 was one of the first series to even offer backlit keyboard.
T440 for example says:
Keyboard UltraNav 6-row, spill-resistant, multimedia Fn keys, optional LED backlight
Thanks buddy!
IBM and Lenovo ThinkPad laptop enthusiasts!