Can you pin down the time period a bit better?

Neither my memory nor my digging now through the Internet Archive Scholar finds a time when environmentalists generally argued in favor of single-use plastic over paper bags.

We know in it wasn't the early 1990s. For example, here's a summary from a 1992 Gallup survey, at https://archive.org/details/sim_reuse-recycle_1992-05_22_5/p... which points out that "More than 3 of 5 respondents, 61%, use paper sacks when asked the "paper or plastic" question" and "More than 7 of 10 consumers surveyed who use or prefer the paper bad cite its recyclable, reusable and biodegradable properties."

Here's a 1992 complaint about the American Journal of Physics using plastic shrinkwrap for its journal, http://digitalcommons.calpoly.edu/cgi/viewcontent.cgi?articl... with the complainer saying "In the current climate of environmental awareness I feel that the physics community should lead by example. The AAPT should adopt, or if necessary develop, an environmentally more appropriate mailing method."

The editor replies that the calculation is difficult and "it is by no means obvious that in this case plastic is bad and paper is good (or at least better)."

Here's an example of the complexity, in a 1992 Chemical Engineering Education column at https://journals.flvc.org/cee/article/download/123883/122917 , which tries to do the analysis and concludes "Plastic sacks appear to result in less atmospheric emissions and require less energy. On the other hand, paper sacks rely on a renewable resource for material and energy . Thus there is no clear, environmentally superior product . The consumer is left with a difficult choice, and as illustrated in the problem this choice must be made with incomplete information."

A 1999 newsletter at https://web.archive.org/web/20200226112831/https://rucore.li... suggests not using a disposable bag, but to use paper if you know it will be recycled: "Best choice is reusable cloth bags which used just 5 times will displace all the pollution caused by the manufacture of a disposable bag. But face it, most of us forget. Next best choice is recycled paper especially in a community like ours that recycles. Polyethylene bags require less energy to manufacture and cause less damage to the environment than do paper bags when paper is not recycled."

This 2003 New Jersey Sierra Club newsletter at https://web.archive.org/web/20200210133922/https://rucore.li... , starts by pointing out "paper or plastic is not a very important decision. ... Topping off the list of consumption choices that matter most to the environment are TRANSPORTATION CHOICES", followed by "WHAT WE EAT" and "VARIOUS DECISIONS WE MAKE IN OUR HOMES". As for the paper-or-plastic question, "there is little difference between choosing paper or plastic bags. So choose whichever you prefer, or choose a reusable canvas sack".

Plastic bags kept getting cheaper to make, and by 2009, http://www.ijqr.net/journal/v3-n4/08.pdf writes: "The "paper or plastic" conundrum that vexed earnest shoppers throughout the 1980s and 90s is largely moot today all over the world. Most grocery store baggers are not bothered about it anymore.".

On the other hand, the downstream environmental effects aren't captured in production costs, which is why "There is a growing international movement to ban or discourage the use of plastic bags because of their environmental effects. Countries from Ireland to Australia are cracking down on the bags and action has already begun in many countries."

That paper then argues "The answer to the "paper or plastic?” dilemma is: Neither. They're roughly equal in pros and cons. While convenient addictions, they both gobble up natural resources and cause significant pollution. May be plastic is a little less harmful than paper." but commenting that you should reuse plastic bags, and "we must not forget the tradition of using cloth bags, which are reusable, washable and durable, pollution free and therefore environment friendly"

This is in accord with my memory, where environmentalists have consistently said to use reusable and washable cloth bags over disposable bags.