With this question I intend to clarify whether the game situations in which "infinite loops" are created can be considered legal; and, if they are not, if there are formats in which they are legal.
The example that can be built is that of the - well known - combo consisting of the Splinter Twin and Deceiver Exarch cards, which today is no longer possible to perform, due to the removal of Splinter Twin from the Standard and Modern formats, in which it is no longer legal.
A reconnaissance of the combo follows.
- If you already have a Deceiver Exarch on the battlefield from the previous turn, you can enchant it with Splinter Twin;
- tapping the Deceiver, it is now possible to produce a token copying the Deceiver, and with Haste ability;
- then, if the opponent does not play with the white color (or, if he does, hoping that he does not have a Disenchant in his hand with which respond to!),when the token enter the battlefield ,it is possible to choose one of the two triggered abilities of the just-created Deceiver-Token;
- choosing to untap a permanent, the first Deceiver will be targeted (the original non-token one, the one enchanted with Splinter Twin), which in this way can be tapped again to produce a new token equal to the Deceiver;
- You can repeat this process indefinitely to produce as many tokens as you want;
- then,it is possible to attack the opponent to win the game.
Hoping that all the above is correct (if not, please tell me where there may be a misunderstanding), I think the procedure in question is able to be a good example of what I mean with the expression "infinite loop". Or, in any case, I think it's a good way to define it:if there are better ones, a mention of them is welcome in the answer.
Since this operation is today no longer possible - due to the removal of Splinter Twin from the two most recent formats - I wonder if this may be due to the possibility that this card has well demonstrated in producing "infinite loops".
The reason for a possible ban of this kind of game actions may be due to the fact that, in order to make this action legal, one should be able to physically perform the "infinite loop": what is however impossible. Even if, obviously, the operation may be stopped at a certain point, for example by producing 300 tokens, of course, and therefore avoiding a real "infinite loop".
But there remains the obvious possibility, within this combo, of being able to make an infinite loop in this way.
If the removal of the Splinter Twin card is due to this (and this reason only) then I wonder if - in Modern and Standard formats - other operations that can produce "infinite loops", or that in any case can be associated with them through the use of various cards:
1) are considered legal game actions; or
2) are not considered legal game actions.
In other words: are "infinite loops" legal or not?