Apple Notes lets you password protect certain notes, expiring access fairly quickly (on mobile, verification is of course via Face or Touch ID). I use this for notes containing particularly sensitive financial and medical information. Concern is not so much re: hackers as in-home snoopers and looky-loos. Especially on mobile (I frequently let guests, friends, and family use my iPad).
I realize not everyone needs this, but I’d imagine it’s easily implemented and ought to be non-intrusive. And for those requiring it, it’s kind of a gaping lack in Obsidian (which seems great, btw…I just discovered it!).