I know that mediatek doesn't share their source code (you can buy it), and what I was wondering is does any MTK device gets updates when newer Android version rolls out, and not just new versions, just does they get software updates? I asking about stock ROM-s made by phone manufacturer. Mostly interested about LG devices.
Sure - devices from Xiaomi are good examples.
Redmi Note 2, released in 2015, still gets stable and weekly development builds of up-to-date MIUI8. Whether the Android version beneath the UI was updated is a question, but that also stays true for other MIUI devices regardless of the SoCs they're using, and you're not asking about Android version updates exactly, either.