Actually, there was a time before 1950 when slacks on JW women at any time was frowned upon. Here are 2 QFRs that shows how the WTS talks out of both sides of their mouth. The final quote in 1984 shows how the writers are cutting and pasting. They put the responsibility of allowing slacks or not in the congregation/field service on the local elders in each congregation which accounts for the varying applications.
WT 10/1/51 p 607-8
When
Deuteronomy 22:5 says that a woman should not wear a man’s clothes, does it mean that women should not wear slacks?—J. P., Pennsylvania.Deuteronomy 22:5 (Da) reads: "There shall not be a man’s apparel on a woman, neither shall a man put on a woman’s clothing; for whoever doeth so is an abomination to Jehovah thy God." This text certainly was not recorded with the thought in mind of preventing modern women from wearing slacks. Men did not wear slacks or trousers when this was recorded, but what we would view as dresses today. In parts of the Orient, in fact, the men wore dresslike robes and the women wore pajamalike trousers of varying styles. So the wearing of slacks or even work pants, such as around a farm, is not forbidden by this text and is an individual matter. The women can use good judgment as to time and place and what is accepted as proper in the section where they reside. In some sections where winters are severe many women wear trousers or ski suits or some similar garment that covers and protects their legs. Such is not Scripturally wrong.
At Deuteronomy 22:5 the Bible is not dealing with fashions or fretting over styles, but apparently it is here forbidding persons of one sex from wearing the clothing of the opposite sex for purposes of deceit, to appear of the opposite sex, to hide the true facts. Men should not try to deceitfully dress like women to hide the fact that they are men, nor should women try to dress in men’s clothes to hide the fact that they are women. Being more specific, the Bible seems to be striking a blow against the sin of sodomy. It was a disgrace for a woman’s hair to be shorn like a man’s, and it was a dishonor for a man’s hair to be allowed to grow long like a woman’s. (1 Cor. 11:6, 14) The woman was not to appear masculine by having short hair like a man’s or by wearing clothes like a man’s. It might suggest to others that she was available for unnatural sex uses. Likewise the man. If he wore long hair like a woman’s or garbed himself in women’s clothes he would certainly appear effeminate and open to propositions from men for unnatural sex use. So it is this deeper meaning with sodomy in view, and not a mere switching of clothes in itself, that brings this practice under prohibition and makes it deserve the severe judgment: "Whoever doeth so is an abomination to Jehovah thy God."
11/1/72 WT pp. 671-672• In view of what is written at Deuteronomy 22:5, is it proper for a woman to wear slacks?—U.S.A.
Deuteronomy 22:5 reads: "No garb of an able-bodied man should be put upon a woman, neither should an able-bodied man wear the mantle of a woman; for anybody doing these things is something detestable to Jehovah your God." This text is not discussing styles of clothing. The prohibition concerns one’s putting on things specifically designed for the opposite sex.
The distinction between the sexes is of divine origin and the law set forth at Deuteronomy 22:5 served to preserve that distinction. When it comes to appearance and attire, the usual thing is for a man to want to look like a man and for a woman to look like a woman. For an Israelite to have acted contrary to this deep internal sense of what is fitting could have led to homosexuality. Thus the law at Deuteronomy 22:5 also opposed this sin.
At the time the law was given, both men and women wore robes. But there was a definite difference between the garb of men and that of women. Similarly, in some parts of the earth today both men and women wear slacks. But styles of slacks for women differ from those for men. Accordingly, the principle taught at Deuteronomy 22:5 would not rule out a woman’s wearing slacks or pants.
Moreover, Christians are not under the Mosaic law. (Rom. 6:14) Insistence on applying the letter of this law would therefore be contrary to Christian teaching. So if a woman were to put on a worn-out pair of her husband’s trousers to do a job around the house or on the farm, she would not be going against the evident purpose of the law, namely, to prevent confusion of sexual identity and sexual abuses.
The fact that Christians are not under the Mosaic law but are guided by its principles calls for them to use discernment, good judgment and to exercise their conscience. A Christian woman appreciates that whether it would be proper for her to wear slacks or pants depends upon factors other than her personal likes. She would not want to be the cause for stumbling others or bring reproach on the Christian congregation. Clothing that may not be looked on with disfavor if worn in the privacy of one’s home or at work may be objectionable if worn at Christian meetings and when publicly proclaiming God’s Word or carrying on other public activity. Attitudes, too, may differ from area to area. The Bible’s counsel is that women "adorn themselves in well-arranged dress, with modesty and soundness of mind, . . . in the way that befits women professing to reverence God, namely, through good works."—1 Tim. 2:9 10.
7/15/84 WT p. 25
The principle in this text would not rule out a Christian woman’s wearing slacks sometimes, as when working around the house or on a farm (notice this does not mention being in public such as shopping). And according to local custom (as decided by the elder body) and necessity (again as decided by the elder body), slacks may be the desired attire in very cold climates.