4) sounds like a recipe for disaster if the product is large enough. Nobody can remember everything. Heck, I start writing a text document when something gets too big for myself.
Pivotal R&D has struggled with maintaining context. For multi-megaSLOC codebases with many moving parts, organic memory begins to fall a bit short.
Different teams deal with this differently. Some write design notes, some keep architecture design records. But IMO it's one of our weaknesses.
Also a problem that's solved by the combination of pairing and frequent communication. You don't need full context on anything yourself, as long as the combination of you, your pair, and anyone you can grab within arms reach can fill in the blanks.
This practice is just saving time writing down behaviour that won't be implemented.
What do you do when people forget things?
There's still something written down, just not massive amounts.
Personally i try break stories down smaller to avoid the need for large tasks which helps. I also try minimise the time between story creation and story development.
Otherwise I just communicate a lot.
Things get differently from what I invisioned, sometimes they're worse sometimes they're better.