First of all, there are some topics about this already, search is your friend.
For which routers are near EOL only @alzhao knows.
HOWEVER, from a consumer perspective, new products will always have the most support. Since GL has only a handful of firmware engineers, new products get updates first, then the most popular products, and then the rest of the products, mostly by age.
So the question is not really if GL will update the products or not, its more of a “how often” will they update the products. A lot of users have been frustrated in the past that updates come out very slowly for the older products, but there is not much that can be done for that. Looking at the testing firmwares, products have been getting updates well after 2 years after EOL, so that is around the average you can expect, 2-3 years updates after the last one is produced (if we just look at their current track record, how that is in the future again, only @alzhao could say).
So which are the most sold / popular? Mango most likely of the mini routers, and since there are no new models of the mini routers, then those will probably not EOL any time soon. Other popular products are probably the AR750s. In general any router that has full support in vanilla OpenWRT will get a lot of updates since it is fast to do, without having to do a lot of porting and so on.
Unfortunately some routers have gone EOL quite early, like the Brume-W and USB150, just because the chips in those products are either very hard to find, or just not being produced any more. The “normal” Brume without wifi is however not EOL, so those updates will come for a long time for Brume-W as well since they are practically the same.
So personally if i were to get a router, i would always get one of the latest models. GL doesn’t have a list of release dates for routers, but a quick way is to go on the FCC site and just see which routers are the newest:
If you want to have all the latest features, then the newest routers will be first with that. For a travel size router, that would be Beryl for the most powerful one, Opal for a replacement of the AR750s in terms of power. For the mini routers it would still be Mango until a new mini router is released.