Banishment renders you unconscious which loses the concentration sending you back from where you were banished from i.e. the maze 

Some points that I think are being missed in previous comments.

1. The Banishment spell only renders its target incapacitated if they are on their home plane when the spell is cast. The section about a target native to a different plane than they are currently on doesn't mention the condition.
2. If a DM decides this trick doesn't work, then it doesn't work! The DM is the arbiter of rules and gets to make those calls. I could lawyer speak this in either direction, but in any game of D&D the DM gets to decide how they will interpret the rules to ensure the game is fun for everyone.

This spell is super weird, and a bit underwhelming for an 8th level spell.

It does have some interesting features though. First is the lack of a save to initially send the target to the maze, meaning you're guaranteed to rob it off at least one action (unless your concentration is immediately broken). Second is the fact that escaping is a check, not a save, so Legendary Resistances can't be used to overcome it, and unless the creature has exactly the right spell (which it would need to cast first) there aren't a lot of ways to brute force it.

The downside though is that unlike banishment whose duration is up to you after the initial save, the enemy gets to test every round on this one, so in the worst case scenario it comes back on its next turn having lost only a single action; so it can still use any bonus actions, reactions and Legendary Actions that it has.

On the other hand, DC20 on an Intelligence check isn't something that most enemies are good at; even the smarter enemies may only have INT +4 or +5, with few (if any) ways to boost it further, so it's not an easy check for many enemies, and may even be a straight 1 in 20 for some.

Even in the worst case I guess the benefit is that robbing an enemy of even just one action may be all you need; it gives you a high level "oh crap" button that might give your party enough of a respite to heal downed players, and setup a few attacks ready for the creature's inevitable return. That's not bad at all, but it still feels underwhelming for an 8th level spell.

I dunno, I guess I just feel like given the flavour there should be more to it, e.g- the creature and the caster locked in a battle of wits as the creature tries to escape over several rounds of contested rolls, so it would be guaranteed to last longer but the trade off is that you've tied up your wizard who has to focus on the maze while the rest of the party scrambles to do whatever needs doing?

Can encounters happen in the maze?

Would Cleansing Touch end this spell? 

Banishment only incapacitates someone if they are native to the plane you banished them from. Otherwise they just appear somewhere in their home plane.

I doubt it since you have to... You know, touch them. And they're in a different plane of existence so that would be impossible.

Here is my take. 

This is a good spell to use against players, it's temporary, they still get actions, and it can be broken by other party members targeting the caster. 

It's also an opportunity to scare the players a bit, as you can make the maze creepy or horrifying while they they are in there, for instance by making them feel like there is something stalking them in the maze, traces of blood, bits of fur or previous victims etc. 😄

Many monsters have a -1 to their Intelligence checks, which means their chance of escaping with their action is exactly 0%, meaning they're gone for the full duration of the spell. Out of the 297 CR 17+ monsters, 48 of them have a 0% chance of making the check; 39 have a 5% chance; 13 have a 10% chance; 25 have a 15% chance; 33 have a 20% chance; 29 have a 25% chance; 55 have a 30% chance; 20 have a 35% chance; 12 have a 40% chance; 13 have a 45% chance; 1 has a 50% chance; and 9 have a 55% chance.

This means it's likely to rob a big bad guy of multiple actions without a saving throw. Also; if you have someone else use feeblemind on them, their chance becomes a straight 0%. By then, combat is over, and the enemy is singled out.

Can this be used on yourself? I can see a scenario where you may be the last one standing, and this being one of the few options left you have. Could use the 10 minutes to down some healing potions/send messages (if possible?) to other allies etc.  And, depending on the fight/situation, could lead the enemies to believe you've just yeeted yourself away ala teleport or something. After 10 minutes id imagine they wouldnt be just hanging around?

I mean you could, but why not cast teleport? It's a lower slot, you don't have to hope that they'll leave in 10 minutes, and could even save more than just yourself 

I'm not intending this to be my first or even third option of escape/get out of danger. Just exploring whats possible.

Forget Feeblemind, use Hex. A first lvl spell that has no save and gives disadvantage on one type of checks (INT) for one hour.

Only downside is concentration.

It automatically gets rid of a Tarrasque for 10, even with a Natural 20 their Int score is too low to escape.

I just imagine a player ragdolling aginst an invisble skybox.

*-1 int*

I was playing as a barbarian with -1 int when this got cast on me. needless to say, TPK.

it had advantage on con too..

Can't you just use plane shift or demiplane to get out

Quote from Bhuraelea >>

Forget Feeblemind, use Hex. A first lvl spell that has no save and gives disadvantage on one type of checks (INT) for one hour.

Only downside is concentration.

 so if you are using the 10 rule that is a -5 + there int if you are fighting tiamat with a level 7 hex she will get an 18 most saves at level 20 are 19 so that is how you get rid of tiamat