Just when you thought that Windows Phone 8.1 was dead and buried, along comes a reminder that it's still part of a major ecosystem. In this case, running HERE Maps (/Drive+) - Windows Phone 8.1's HERE clients still (in 2021) have access to HERE's servers and APIs, it seems. Checking my Lumia 1020, I just got the best part of a Gigabyte of brand new maps for England - and, no doubt, similar where you are. Oh, and the live traffic all still works too. Impressive.
The updates are, of course, due to HERE being a huge map supplier still, gathering and updating maps for a great many clients, not least on cars with HERE-enabled in-vehicle satnavs. And the APIs that let these get new maps still works for 'ye olde' Windows Phone 8.1. Happily.
To check for new maps on your old Lumia, go to HERE Maps and then tap on '...' in the bottom right corner, choosing 'Download Maps', then 'Update current maps'.