Do you use an apostrophe for possession in decades--not for plurals, just possession, as in "a 1960s style," that is, "a style of the 1960s"?
Thanks, everyone, for weighing in. I asked this same question on CMS Twitter and someone from the style manual responded that I should refrain from using the apostrophe. The reason is that CMS considers "1960s" in my example to be an adjective, not a noun. Since my clients--university publishers--tend to use CMS style, I'm going to go along with CMS's suggestion. I should point out that this is not actually found in CMS manual; it appears to be a response discussed around the office.
This is not possession. It's actually the use of plural. In your examples we don't need apostrophe. We don't use possession either, the years are not the owner of the style, instead the style took place during those time.
From The New Fowler's Modern English Usage -
Though once commonly used in the plural of abbreviations and numerals (QÇ's, the 1960's), the apostrophe is now best omitted in such circumstances: MAs, MPs, the 1980s, the three Rs, in twos and threes. Except that it is normally used in contexts where its omission might possibly lead to confusion, e.g. dot your i's and cross your t's; there are three i's in inimical; the class of '61 (= 1961).