Agreed, Netgear and Synology have been good with firmware updates.
TP-link is one of the fastest to drop firmware updates, especially when they decide to come out with a V2 of a router. They are good with releasing stable routers and getting everything stable before stopping support, but it also means that you may not get security updates.
For example, the tp-link AD7200 has not had an update for over a year.
As for Netgear, their track record has shown them to release updates for many years after the product is released. e.g., the R7000 got an update on December 18, 2017. The R7800 got an update on December 13 2017.
Many of their 802.11n based routers such as the WNR3500l are also still being supported. though it is important to know that Netgear also likes to drop support for older versions of a router, e.g., the WNR3500L V1 has not had an update in a little over a year, while the WNR3500L V2 had an update on December 8, 2017.