Bountiful Delves loss their status even if you don’t use the key. And after you complete the first tier 8 complete the radiant memory quest you wont the drops until you do.
TLDR
upgrade delves to tier 8 not doing bountiful
do 1 bountiful tier 8
complete radiant memory
do other bountiful delves
That is all.
Thanks.
Also, the TLDR was longer than what you were trying to summarize. But I will now save my Keys until Tier 8 so I don’t waste them.
I’ve read this post 3 times and still don’t know what you’re saying. Can you restate it using other words?
Bro do you even speak Common?
I probably will no be able unlock tier 8 alone for a long time lol. Isn’t that like 605 ilvl content?
I assume M+ are the only players that will be soloing that tier for the next month or so, outside of some specs, paid runs, and cheese.
I already knew this before the expansion launched; I have all my keys saved up for Tier 8 or higher Delves this week.
A delve is bountiful if it’s icon is lit up. once you complete the delve the bountiful status goes away regardless of whether or not you used your key to open the big chest.
Basically once you complete a bountiful delve, even if you don’t open the big chest, the delve is no longer bountiful.
So what you want to do is do non-bountiful delves until you have unlocked level 8, then go do the bountiful delves and use your keys.
This, and the other part of OP’s post is that there is a quest that you get for a new system called Radiant Echoes. You want to make sure you do this quest before you start cashing in your coffer keys on bountiful delves so that you can acquire the new currency.
Eh I have been burning my keys and doing bountiful delves.
I have like 8 keys , I think Ill try till I fail then use the key for the highest difficulty I could do .
There are delves then there are delves with big booty, you want big booty delves to use your coffer key on 
Tier 8 was tuned for 600 in beta. It depends on your skill, the delve you pick, and the class you’re playing, but they honestly are not too terribly difficult at that level. There’s no timer so you can slowly work your way through the fights if needed.
Why T8 or higher though?
The weird thing about it to me is the bountiful chest being ‘consumed’ even if you don’t open it. I feel like Blizzard has the ability to have that not be a thing and have it stay bountiful until either the bountiful delves get shuffled or you open a bountiful chest.
Do I need to complete every Delve before I can unlock higher tiers? Frustrated that I can’t get to Tier 4 when I just completed Tier 3. Wish I could just stand at the same Delve and go up the chain so I don’t have to circle around the map doing all these unlocks.
You don’t have to unlock all tiers of every delve. You can just run the same one all the way up the ladder.
Every week some delves will have the bountiful bonus, and the rest will just be regular. There are special big treasure chests at the end of a bountiful delve (the ones where the icon looks like the doorway is lit up with fire) and these chests give better loot than the regular chests.
However, the loot is still somewhat limited until season one, tier 8. For this reason, people have been saving their keys (which you get from certain epic caches and special assignment world quests) and not opening those chests yet. They want to wait to unlock level 8 where the extra good goodies are first.
Now the OPs PSA is warning people, that while they are doing delves to unlock level 8 (because you have to do a 5, a 6 and a 7 first), to make sure you level on regular delves. If you level in a bountiful delve, even if you try to save that chest for later by not opening it, it will disappear now that you completed the delve…
So, that’s the wordy version I guess. 
Yeah don’t overthink it too much.
Smash the delve(s) you like doing, use your keys if you can use an upgrade, and repeat.
Wordy, but clear and informative. Thank you
T4+ unlocks today, you previously were capped at T3