The rubrics say that the priest washes feet on Holy Thursday (if he elects to do this). Can/should deacons assigned to the parish also wash feet along with the priest?
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The rubrics do not and never have assigned deacons to the task of washing feet. It has remained the responsibility of the presider, whether he be a bishop or a priest.
A lot of parishes take a lot of freedoms with the foot-washing, probably because it is optional, which gives it a somewhat flexible feel. I’ve explained all this in Glory in the Cross, but I always come back to the belief that the rubrics have it right: The presider, usually seen in the role of leader, deliberately and humbly takes on the role of servant. On his knees before his people.
In my view, it would be more fitting for the priest to wash the feet of the deacon than for the deacon to wash the feet of others.