JOEL - Here are some statements I found on the subject of mission farewells. This first one was actually restated over 30 years ago about a policy that is now almost 36 years old.
Farewells—“Since September 1966, it has been Church policy that no missionary farewells be conducted in sacrament meetings. Failure to observe this policy in some areas makes it advisable to reiterate the policy concerning the recognition of departing missionaries: (1) The bishop or branch president should invite the departing missionary to speak in sacrament meeting, not as part of a farewell program, but simply as a sacrament meeting speaker concerning whom no special notice, publicity, or fanfare is given. (2) Farewell program leaflets and newspaper notices are not to be printed. (3) Collections are not to be taken up at the door; however, members may well be encouraged to contribute to a ward or branch missionary fund from which missionaries are assisted with travel and other expenses. Individuals are free to make personal contributions directly to missionaries. (4) Bishops and branch presidents should counsel families against holding receptions or open houses for departing missionaries; accordingly, announcements in church meetings of such receptions or open houses would be out of order.” (“Policies and Procedures,” New Era, Apr. 1971, 50)

And here's the information about the most recent one that you are probably referring to:
"While departing and returning missionaries may be invited by the bishopric to speak in a sacrament service, the focus of the meeting remains on our Father in Heaven and on the supernal gift of the Atonement wrought by his Son, Jesus Christ. Departing or returning missionaries, along with others who may be invited to speak or perform a musical number, should concentrate their message on the doctrines of the gospel of Jesus Christ as taught in the scriptures and by Church leaders. Neither missionaries nor their families plan sacrament programs. If invited by the bishopric, they may make suitable suggestions; however, Church policy states that bishoprics are to plan sacrament meetings and see that they are conducted in a reverent manner. In a recent announcement, the policy regarding the participation of departing and returning missionaries in sacrament meetings was again emphasized:
“Bishoprics may honor newly called full-time missionaries by asking them to speak in a sacrament meeting before their departure. The bishopric plans and conducts such meetings and invites those who will participate. Family members of the missionary may offer prayers, present special music, or speak. Talks and music should be worshipful, faith promoting, and gospel oriented. The regular time of the sacrament meeting should not be extended. This policy also applies to returning missionaries."(Ensign, Nov. 1994, 112).
Members and local leaders should avoid practices that may detract from the sacred nature of a mission call or create unnecessary expense, such as holding open houses for missionaries (except for family gatherings), sending [or publishing] formal printed announcements or invitations, printing special programs, or forming reception lines at the meetinghouse after the sacrament meeting”
(“I Have a Question,” Ensign, Sept. 1996, 60 - Response by Hoyt W. Brewster Jr., president of the Netherlands Amsterdam Mission, former managing director of the Church’s Priesthood Department)

The way I remember it, mission farewells were big productions, with a special program printed up with the missionary's picture. The entire Sacrament program and content was centered on the missionary, with a lavish open-house following the meeting. Before and after the meeting, the ward clerk would sit at a table in the lobby and accept donations for the missionary. These are the things that were done away with when they came out with these statements. To preserve the sacred nature and importance of the Sacrament meeting, now the missionary is only asked to be one of the speakers and he is given a topic to speak on by the Bishop. Any other invited speakers keep to the same subject in their talks. How closely a ward adheres to this policy I am sure varies somewhat throughout the church.
I remember that another reason for the change was that, at this time(1966-1971), there were also a lot of young men entering the military or getting drafted and going off to war. But nobody was making a big deal about them leaving to fight for their country, and it left them feeling unappreciated.

The announcements you are referring to are sent out only to Bishops and Stake Presidents; they are not made available to the general public.
I think there are many things that have been perpetuated in the church simply by traditions that started long ago. Many members may confuse these traditions with actual doctrine or church policy, and because of this it is hard for them to accept this kind of change.