I have spoken with ASUS and they have now added the latest firmware for the DSL-N66U to the original download link/set.
ASUS will continue to provide updates for this router model, but had assumed no demand and hence removed from list.
DSL-N66U_9.1.2.3_783-g4cb10eb.trx.zip
Edit: ASUS have just given me this direct link to the latest firmware for the DSL-N66U
DSL-N66U_9.1.2.3_783-g4cb10eb.trx.zip
https://www.asuswebstorage.com/navigate/a/#/s/1EB2A125096D4BA893AFD7AFDC9DC2EFY
In a completely sane world, madness is the only freedom (J.G.Ballard).