I live in a really dry climate, so I don’t really notice issues unless I have a spool out on the printer for months, and even then dust is more of an issue than humidity.
To be safe, I bought a storage bin with a gasket on the lid that I store all of my other spools in, and that has seemed to work great.
I've submerged PLA in water overnight and then printed it to prove humidity has no effect on PLA.
PLA doesn't give a shit about humidity and never has. All the early days of 3D Printing that used Nylon needed drying out.
Even early days PLA getting brittle was about strain fatigue on the filament and nothing to do with humidity. But modern day PLA these days doesn't suffer from those old school issues anymore.
Likewise, I have had PLA get brittle and snap on the way to the extruder when left loaded on the printer and exposed in my basement for extended periods, but this behavior disappears after I bake it in the filament dryer, even on the same spool.
I've had bad stringing before with PLA, drying it made it go away completely with the exact same print settings and model.
I'd argue that the vast amount of (recent, not only older) resources available online showing similar experiences to mine disproves your statement that PLA has no issues with moisture retention.