In the Armenian Church, at the beginning of the Eucharistic part of the liturgy, the deacon calls out: "Let none of the catechumens, none of little faith and none of the penitents or the unclean draw near to this divine mystery."
In the old days, not only catechumens, but also those who were in a state of penance, would then have to leave the nave (I think that's the right word) and go into the narthex. I know someone who once saw an ancient stone church in Armenia where the narthex was actually larger than the nave, since the practice of excluding the penitents used to be taken so seriously.
Excluding catechumens and penitents is no longer done today. I can't tell you, though, when the practice fell into disuse.
As a footnote, a custom unique to the Armenians may have started because of this old practice. If you go into an Armenian church during Lent, you'll notice that the curtain in front of the altar is closed during the entire liturgy and that no one takes Communion. This custom is so old, no one knows for sure how it came about, but one theory has to do with the old practice of excluding catechumens. The theory is that because the Church used to baptize catechumens at Easter, they would be allowed to stay in the nave during the entire liturgy during Lent, as a way to prepare them for worship as full members of the Church. However, because catechumens really shouldn't be looking upon the altar, and because they can't take Communion, the curtain would be closed when they were present and Communion wouldn't be given. (Of course if a member of the Church wants to take Communion during Lent, they really can. It's not like there is a ban on it, or anything. It's just not customary.)