The lifetime Birdseye product is new and I have not seen any specific terms and conditions. But their policy on lifetime City Navigator maps is well known:
https://www.garmin.com/en-US/legal/lmdisclaimer
"you will receive map data updates when and as such updates are made available on Garmin.com during the useful life of 1 compatible Garmin product or as long as Garmin receives map data from a third party supplier, whichever is shorter. A product’s “useful life” means the period during which the product (a) has sufficient memory capacity and other required technical capabilities to utilize current map data and (b) is capable of operating as intended without major repairs. A product will be deemed to be out of service and its useful life to be ended if no updates have been downloaded for such product for a period of 24 months or more."
There was a lot of hang-wringing about this in the past, but I don't recall any credible examples of Garmin pulling support for map updates. So, I think that language is largely "CYA" from their legal department.
I can say from personal experience, that they did not cancel my lifetime maps when I waited more than two years between updates on two of my devices. I am also unaware of anyone who stopped receiving updates because they had exceeded the "useful life" of their GPS. Have never seen an example of them pulling updates because the map supplier changed either (because it hasn't changed).
I can think of one vocal individual who claimed they did this with a subscription he bought for a GPS that is at least 15 years old. Then later, the same person said he could download updates again. If anyone here has experienced Garmin refusing to honor a lifetime subscription, I'd like to hear about it.
AFAIK, you cannot purchase a lifetime maps subscription seprately anymore. In the past, there was a way to get a lfietime subscription if you purchased City Navigator maps on DVD, but Garmin doesn't sell maps on DVD anymore. All of their automotive devices include lifetime subscriptions now. You might be able to purchase a subscription if you had an old automotive device that did not originally include lifetime maps (although that would make no sense). But that subscription would be an update to the maps that were pre-installed on the GPS, as opposed to a map that you purchased separately.
Garmin only sells maps as downloads or micro-SD cards with a pre-instlaled map now. Neither of these products is eligible for updates. You have to just buy a whole new map if you want a newer version now. Garmin has never offered any form of updates for their topo map products, and does not update them very often.
With Birdseye, a normal subscription only lasts a year, but you can download all you want during that time and it never expires (you can continue using it after the subscription ends). It is, however, locked to the original GPS for which it was purchased. I read a report from a new owner of a GPSMap 66, and although you are supposed to be able to download Birdseye directly to the device (with wifi) forever, he said in Basecamp that it claimed his subscription was for only one year. That's probably just because the lifetime Birdseye thing is new, and because Basecamp is not being updated. So it's possible that he will only be able to download with Birdseye for a year, but will still be able to download on the GPS afterwards.