Like for the previous question...yes ofcourse you can use mass amnesia, even without saving throw, if you are the GM.
The point is, in the same exact way for every spell that can mess up players characters and players experience is to do something like this with a plan in mind.
What happen the second after the spell take his effect? I give you some advices:
Don't let them roam without a purpose, be proactive and create a story focused on this event.
Don't do something like this if they can recover just with an heal spell. You are wasting their time and your time.
Don't use wish. Wish is a spell that players can use and an effect like this is clearly out of wish scope so don't use that. Use rituals. Use a forgotten book with inside a powerful curse. Create something, an artifact, what do you want but do not use a regular spell from a regular book if you want to create a unique effect.
If you want such a powerful effect and such a negative condition to happen you need to be prepared. This event must (MUST) be fun for your players. Not boring. Not tedious.
I give you an example. In your same exact situation (right before the final boss fight) i want to show the power of the boss (not the pit fiend, the final boss do something like this) and i decide to kill all of them without saving throws. They just die with a dramatic description of their deads.
What happen next? They wake up in a strange realms (maybe the realm of dead, maybe not) and maybe they are ghosts, maybe they are souls, and they start to discover what the hell just happened talking with other souls/ghosts/whatever. They have been killed by the most secret and powerful ritual ever created by mortal hand...a ritual their enemy created long long time ago. They are trapped forever, dead but stucked, and there is no way to escape...except for a rumour...or a legend...or maybe something strange in this realm like a lake, a mountain, something.
What am i doing? I'm creating a quest starting from this event. They will fight in this realm? Maybe, maybe not. They will level up? Probably yes. They will find/obtain something cool ? Ofcourse yes.
How long it should be? 2/3 sessions max.
Remember they have a boss to fight and they want to fight him.
And after they escape? They are now immune to that ritual! So they will fight the boss and he will not have is secret weapon to use against characters.
With your mass amnesia build up a story, this is what i'm saying. If you don't want to spend time to create a fun experience for your players...don't do something like that.