The answer
If a deacon does not take Communion, he is not supposed to wear the tunic, because it is a special garment for serving at the altar. So the deacon is not allowed to serve at the altar and not take Communion.
From the point of view of taking Communion, there exists no difference in the Church's rituals between a deacon who serves outside the sanctuary or inside it. They are all deacons and are supposed to be prepared to receive Communion, otherwise they would be a bad example to the people.
The only reasons for not taking Communion are: not having fasted, not having repented, or not being spiritually prepared, all of which also prevent one from serving as a deacon. So anything that disqualifies one from taking Communion also disqualifies one from serving as a deacon.
The whole congregation is supposed to come to church in a state of fasting, and spiritual preparation, because as the Reciter says in the Psalm: "Holiness adorns Your house, O LORD . "(Ps. 93:5)
In the past, all those who attended the Holy Mass, the Mass of the Saints, had prepared themselves to partake so there is all the more reason now for the deacons to attend and to wear their tunics!
For a deacon to attend just to sing the chants and then leave, this is something not permitted according to Church rules. If he doesn't wish to receive Communion, or is not prepared to do so, then the priest ought not to sign the cross over his tunic.
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Thrice-Blessed Pope Shenouda III of Blessed Memory