Actually, it is still required for seminarians to receive the instituted ministries of acolyte and lector prior to their ordination to the transitional diaconate (see Canons 1035 and 1050).
Since an altar server and commissioned extraordinary minister of holy communion can fulfill many of the roles of an acolyte, it's optional for lay men (as it is reserved to men alone) to receive this formal institution prior to serving in these roles. And, in fact, since the Vatican II reforms limited the reception of this ministry to men alone, most dioceses do not make use of this ministry other than that their seminarians are instituted into it.
All this means, is that in practice, in most places, acolyte remains a "step" to the priesthood as it would have been considered before Vatican II as a minor order. This, by the way, is largely the reason why these two ministries are conferred on seminarians in their seminary rather than in their home diocese--so as to not confuse the lay faithful (all of whom are commissioned lectors and commissioned extraordinary ministers of holy communion). Of course, there some notable exceptions in the US: some dioceses do use this formal ministry outside of their seminarians.
Some months ago, the Cardinal Arinze, acting on orders from the Pope himself, refused the extension of an indult the US Church had been operating on for a number of years which allowed commissioned extraordinary ministers of holy communion to purify the vessels. Since the indult is no longer in effect, commissioned extraordinary ministers of holy communion should no longer be purifying the sacred vessels.
However, the law itself permits the instituted acolyte to assist in purifying the vessels (General Instruction of the Roman Missal, no. 279). So I personally wonder if we will see more lay men receiving this formal ministry at the request of local bishops in order to assist their priests in purifying the vessels.
Here are the canons that were mentioned:
Can. 1035 §1 Before anyone may be promoted to the diaconate, whether permanent or transitory, he must have received the ministries of lector and acolyte, and have exercised them for an appropriate time.
§2 Between the conferring of the ministry of acolyte and the diaconate there is to be an interval of at least six months.
Can. 1050 For a person to be promoted to sacred orders, the following documents are required:
1° a certificate of studies duly completed in accordance with can. 1032;
2" for those to be ordained to the priesthood, a certificate of the reception of the diaconate
3° for those to be promoted to the diaconate, certificates of the reception of baptism, of confirmation and of the ministries mentioned in can. 1035, and a certificate that the declaration mentioned in can. 1036 has been made, if an ordinand to be promoted to the permanent diaconate is married, a certificate of his marriage and testimony of his wife's consent.
I stand corrected. Thanks for the comment.