(Yet another) follow-up from this question:
As someone mentioned in the question above, some 'friendly' spells (Calm Emotion, IIRC) specify that one can choose to fail the saving throw. In this case, the allowance makes sense, given that the spell has both a 'defensive' component–removes fear, charm–and an 'offensive' one–reduce aggro/threat, if you will. However, Banishment, and most other spells that require saves, are (meant as) offensive spells, so, it wouldn't make sense to allow the target to choose to fail the save–at least not explicitly. Can someone choose to fail a saving throw even for these spells?
The specific case that comes to mind (as per the question linked above) is using Banishment to return yourself to your native plane: if you are somehow transported to a different plane, casting Banishment on yourself, failing the throw, and then maintaining concentration for one minute should return you permanently to your native plane.
Considering non-spell saves/'defenses,' it seems that one can choose to fail (insofar as verisimilitude is a factor): a warrior can drop his shield, stand still, and allow his/her enemy to strike them down (effectively setting their AC to 0, so to speak, or even adopting/mimicking the Incapacitated condition); a PC making a strength check/contest can choose to give in/stop exerting strength; and the infamous/contentious issue (at least, in 4E I believe it was so), any character can presumably choose to hurt themselves with their own weapons, even if the PHB doesn't have a rule for it.
Best Answer
On March 10, 2016, Jeremy Crawford tweeted, "No rule lets you opt to fail a save. As DM, I might allow it, assuming you aren't incapacitated or dominated." Mike Mearls also said that he would allow it (as a DM).
Willingly failing a saving throw is a house rule that the designers are okay with, but it is a house rule. Strict RAW does not allow it.