BiS exists with traits of azerite armor, since it can't forge. BiS also exists in a "general" sense, in that there will always be a BIS pair of gloves if 3 bosses drop gloves but only 1 has your best stats. Of course there is a good chance it won't remain your BiS, and there will never be a 100% set full gear loadout due to forging, but you can still have bis.
*Insert every single ridiculous PC parts detail here that no one cares about*
BiS is the best possible item you can get regardless of what you are currently wearing. The point of BiS is to aim for them and farm them to get them all. What you wear currently is irrelevant and the guy who told you to get helm2 is an idiot who most likely think that sims mean something when none of the fights in the game look like sims.
The helm2 is a "good upgrade" and nothing else.
This is interesting. Cause in theory, Helm 1 could be BiS, but ONLY if you have other gear with various stats and/or specific trinkets.
The BiS list can only be complete if you have ALL the gear for it. A gear can be BiS but not yet, cause you would lose too much Haste etc if you equip it and don't have the bis-haste boots.
So yes, a BiS list for every specc exist, but equipping that item MIGHT not be an upgrade until you have more pieces.
Our guild uses BIS as "best in slot, for that raid and difficulty" Ignoring forging and gemslots.
To me, BiS stands for another excuse why your party/raid member wants your piece of loot!
A best in slot setup described a gear set which was strongest during a content period. Originally the focus was set on the best single target output on bosses.
During vanilla best in slot was pretty much unachievable due to the scarce amount of drops or rather it required a very unfair loot distribution in your guild to achieve that. During TBC it was quite common to focus on best in slot items due to better access to items.
With "what-ever-"forged BIS is pretty much gone. theoretically there is still the single best item for each slot (like max titanforged with socket), but in practice pretty much not. Also unlike in the earlier times of wow we have access to so many items that we can build multiple setups for different situations.
This is the sensible approach, we too plan around loot that we're guaranteed to get eventually and not some freak occurrences. Naturally if suddenly a guy gets some lucky 425, then his bis choice in spreadsheet becomes irrelevant, aside from some exceptions like trinkets or set items (ring/cloak set, for example).
We just toss in spreadsheet stuff each of us considers BiS for himself, but if we get better item, we simply remove that from spreadsheet, so we can get proper boss rotation for those that actually needed.