They removed the old firmware from the official website a few years ago, because they launched the improved RT-AC68U, the new 68U has a different CPU (BCM4709), and it is very dangerous to flash this version on a new model.
I have mirrored almost all RT-AC68U official firmware starting from 30043763626, I used to share them on mega, but with the introduction of new models, many people ignored the warning and flashed too low firmware to brick theirs router.
I can provide you with the firmware if you need it, but please make sure your RT-AC68U is A1 or B1 version, flash in other version and you will get a brick.