My question is, do the catechumens still have to leave the service after the litany, or can they remain???
To paraphrase our parish Priest, the call to dismiss the catechumen, like the call to bar the doors, are "dormant" parts of the Liturgy - it is possible that in the "right circumstances" they will be put back into force. Both are hold overs of different situations than we now encounter.
There was a time when it was basically assumed that if you were present for the Holy Sacrifice (the "liturgy of the faithful" or "liturgy of the Eucharist"), you would
be receiving Holy Communion. In fact there is still a canon "on the books" (though not enforced, as should be obvious) that if you go without receiving Holy Communion after "x" number of Sundays, you were to be excommunicated and may only return to the communion of the faithful via certain penances (I forget the number of Sundays, but it was really brief...two or three I think...after that, excommunicated.) Holy Communion ought to be part of the "normal" participation of the faithful in the Eucharist. However
, that also assumes as a necessary condition that one is living and struggling in such a way as to be able
to receive Holy Communion every Sunday. As you can see, there was not much "nominalism" in the early Church, and discipline tended to be pretty strict, and the level of spiritual guidance very thorough. With the mass conversion of entire nations, the Church had to adjust to situations where you were going to have lots of pretty "sickly" people in the Churches, and make a choice - tolerate these people in the hope of saving them, or keep up the high level of rigor as far as ascetical struggle was concerned. The Church chose the former, and basically ceased to enforce certain rules.
Thus, since people who are basically not prepared to receive Holy Communion are present in the Churches Sunday after Sunday after Sunday and are not being canonically disciplined, catechumen are not expected to leave either - since their presence during the Holy Oblation does not imply some typical expectation that they will be given Communion. Also, there was also the expectation that if the catechumen departed, they would not simply go off to do as they please, but would have been taken by someone from the Church to be catechized; rarely outside of monasteries (which as far as I know are the only places now days where the dismissal of the catechumen is enforced) is there anything set up to accomodate this.
The same is true with other parts of the Liturgy which lie "dormant" - such as the call to bar the doors; a memory of when the Church had to assemble clandestinely, so as to avoid the persecution of the devil in his civil servants during the (pagan) Roman persecutions. May it please God to spare us of the need to make hit measure "active" again.