I asked a former FlyBE pilot. For the Dash it is down to company policy and the particular airframe, not all are equipped. From a web search: later batches were ordered without the lights, presumably to save maintenance costs. Maybe the second-hand ones from Republic have them, I'll have to check.
They are hard to spot without those lights.
He thinks that in some aircraft ( like A320 ) it is automatically illuminated when the gear is extended, if not already commanded. I think the EMBRAERs have them as standard, too.
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