I think for GC turn ins they stay relevant, because you can't just turn in whatever you want anyway. At 81, you'll still get the 80 list stuff sometimes, it just bumps the 79 stuff out and replaces it with 81 stuff, and only at 82 you phase out ShB stuff entirely, as far as I remember.
Collectables and leves for gathering should work until 81, at least as far as I've read/remember. For crafting they're shit straight away iirc, 10k xp for everything and like 5scrips or whatever it is for collectables. Have to grab new 80 leves in the EW city and collectables there too(but waste of money doing collectables on crafting at launch).
I would certainly expect Diadem/Ishgard Resto to be shit instantly on both gathering and crafting however. It's catch up content so don't think it'll help with leveling at all, but could be wrong, no precedent here since it's ShB content. Similarly, no idea how Ocean Fishing will go, if it'll still give stupid xp and scale past 80, or if it'll instantly give garbage xp at 80, or if it'll work until 81. If it turns out they scale past 81 I'll look into them but otherwise it's dead content until 6.1.
For my own prep, I've crafted one of every GC item that's possible(79-80 list basically) as HQ, so I can turn in without having to craft unless it repeats for 2 days. I'll sell the shit I don't use, or throw it away. I have plenty of the NQ gathered items in that GC bracket too, no idea how many you'll need for turn ins with HQ removal but probably not that many anyway. I have stupid amounts of GC turn in fishes because I bought them back in 5.0 when they were cheap so I could resell them in 6.0(sadly the HQ probably won't have any additional values, but people still spend good money on GC turn in fishes so they don't have to go catch them themselves, takes a while to sell everything though).
I have only 2 collectable, 2 fishes(80 white scrip shit) because I got them when doing big fishing and figured I might as well keep them in case, if they don't give xp don't really care and didn't need the scrips anyway.
I have 16 gathering leves picked up, the 3x 78 leves for mining and botanist(already completed obviously just need to turn in), and the rest is all the fishing going down to 74 or whatever. I have spare fishes for the 3x 78 ones because if the server goes up at the appointed time, it's 1hour before leve ticks, and I figure I'll just take 3 more fishing leves and turn them in so I don't lose that reset. If the servers are a bit late, I'll just sell them/throw them away no big deal.
I don't know how useful it's all gonna be, but I also have nothing else to do so prepared that over the past couple of weeks because why not.