The new one came with a green bag for me to post back the old one, so I presumed I would need to do that although I've had the old one for several years now (certainly before December 2019).
It was how long you had had the old one. There is no need to send it back if you had it that long. BT are obliged by law to offer recycling of old hubs but you can just dispose of it yourself at your local recycling center when normality returns.
I received mine recently when upgraded my broadband but there was no detachable label with barcode to attach to the green bag supplied to send back the old router for recycling. I will pop it into our local small electrical recycling bank.