Correspondingly, how long is Pathfinder BfA?
2-3 weeks
Secondly, what achievements are going away in Shadowlands? Achievements (and their rewards) are being removed
The Lost Flight quest for killing Heroic N'Zoth, which awards the Uncorrupted Voidwing, will be removed on Shadowlands launch (patch 9.0. 2) Keystone Master achievement for the Awakened Mindborer will be removed in the Shadowlands pre-patch (patch 9.0. 1)
Also to know is, is BfA worth flying?
As to the question of if it's worth it: flying is always worth it, because it makes the world easier to traverse for alts you happen to take through BFA. (And, given the leveling revamp, if you have flying in BFA and you don't in, say, Draenor, it could give you an incentive to continue leveling through there instead.)
Is BfA Pathfinder account wide?
Also, the BfA Pathfinder part 1 meta-achievement is account-wide, meaning that you can complete the required achievements on different characters, although they can't be from opposing factions.