Are the NBTs the same for Minecraft 1.8.9 and 1.15.2? Not you commands, but the NBTs (plural of NBT).
I've only compared the whole thing to villagers now. Yes, they are the same so far. There may be minor variations on some NBT tags, but not most.
Is {SelectedItem: {id: "minecraft: white_stained_glass_pane", Count: 1b, tag: {HideFlags: 1, Enchantments: [{id: "minecraft: unbreaking", lvl: 1s}]}}} correct on 1.8.9 ?
Ufff I don't really know anything about that. Does it work?
I don't know… I'll test it early today
Are you generally not familiar with NBTs?
So generally the nbt data are the same. However, new things were added over time. For example, In the earlier versions for the Armor Stand no marker tag (makes it no longer have a hit box).