I heard that this all originated in the very early Christian days when only the baptized were able to be present during Communion. The custom of having the catechumens leave fell out of use after the fourth century, although the language still remains in the liturgy.
This is the point about 'restoring' this practice. It disappeared over time.
Where does an individual priest draw the line on such 'restorations' before his own vanity becomes superior to the discipline of his Diocese, his responsibilities to his Bishop and his obligations to preserve the deposit of Faith as it has been transmitted to us over the centuries?
Salpy brought up a situation where the dismissal of catechumens was retained as a diaconal proclamation, but no catechumens were actually dismissed. I don't think it's wrong to ask why the formulaic dismissal was retained in all Liturgies long after the custom disappeared. Is it not nonsense to dismiss people who are not present? Why does "Bow your heads to the Lord" get taken seriously, but not "Let none of the catechumens remain"?
We respect the Liturgy as we have received it, and we learn it and love it before we dare to even think about tinkering with it. Even so, I think there is some leeway. If a priest and/or individual catechumens decide to "take advantage" of the dismissal in order to reinstate the practice in that particular parish and supplement their formation, why is it vanity? Sure, it may not be the current and universal practice in the diocese or the wider Church, but this is a local matter. Not every parish has catechumens, and where they exist, it's not wrong to accommodate. Local circumstances often require adjustments to the parish Liturgy that may not be necessary at the parish down the street, but it's not vanity. If Fr X enforces an actual dismissal in his parish, it need not be a problem; it definitely becomes a problem when Fr X or his parishioners try to impose that discipline in someone else's parish.
Our Liturgies all retain certain customs from an earlier time, some of which we no longer follow. It doesn't bother anyone if the catechumens are not dismissed, but it bothers plenty of people if their formulaic dismissal is omitted. IMO, this is an unhealthy way of looking at liturgy. What we do in church is not a cleverly arranged set of incantations and rituals meant to conjure up the proper "vibes". What we do has meaning, and we need to understand it. In doing so, we might "resurrect" certain things which "fell out of use" for long periods of time, but that need not be a bad thing: for example, frequent Communion.