I was reading a meh article today on how the ADA (not "disabilities", but I forget what the D stands for), the law that supposedly prevents government employees from working in the absence of appropriations funding their jobs, came to entail a government shut down. Apparently there was a memo by Garcetti in the early 1980s concerning how to interpret that law, already over a decade old at the time the memo was written. A memo maybe 8 years later added to that interpretation--and the government shutdown was born.
(At no point did the article mention whether there was a prior incident of non-appropriation with federal employees still working. The reason I mention this at all is because two memos interpreting a law allegedly not relevant to the situation were crucial to the current interpretation and implementation of the law.
It's also worth remembering that DACA is just a "memo" with bureaucratic machinery added later to help implement it: https://www.dhs.gov/xlibrary/assets/s1-exercising-prosecutorial-discretion-individuals-who-came-to-us-as-children.pdf
It's not that we don't remember the time when memos carried such weight and consequences; it's just that many things we know about we simply don't remember as being the direct result of memos.