Cfrgtkky

In the comments section of this question the subject derailed slightly: is the word upskirting dependent on today's technology or is it older?

The word upskirt to refer to photographs taken up women's skirts was coined no later than 1995.

The Internet domain upskirts.com was registered on 1995-10-13, and upskirt.com followed shortly after on 1996-06-10.

The term upskirt predates smart phones or other cell phones with cameras. A Washington Post article reprinted in the June 8, 1998, Amarillo (TX) Daily News warns:

In what they describe as a growing trend, police are beginning to catch video voyeurs trying to shoot private parts in public places. These men are aiming the latest compact camcorders up women’s skirts in crowded stores and shopping malls, parks and fairs - and often posting the pictures on the Internet.

. . . [P]olice arrested a 21-year-old man who was holding a palm-size video camera under a woman’s dress. . . [P]olice nabbed a 19-year-old man toting a bulky VHS video camera. He was angling for similar shots in the china department. . .

What began as a small photo gallery on the Internet a couple of years ago has rapidly expanded to more than 40 such “Upskirt” sites. . .

The first example of upskirting quoted by the OED is from 1998. And it would appear that it does not predate digital photography.

The habit or practice of taking upskirt photographs or videos. See
upskirt adj. and n.

1998 Re: Lets start Interesting Post in alt.native (Usenet
newsgroup) 12 Sept. Thus begins the fine distinctions of ‘privacy’
rights in public places, of which the issue of ‘upskirting’ is just
one.

2000 Atlanta Jrnl. & Constit. (Nexis) 10 Oct. a12 Beginning today,
Ohio law increases the penalties for secretly taking pictures up a
woman's skirt.., called ‘upskirting’.

2005 West Australian (Perth) (Nexis) 17 Feb. 38 He would hide the
camera inside a backpack..and..film up his victims' skirts (known as
upskirting).

2012 C. Quigley & D. Pollock What on Earth are you Wearing? (new
ed.) 97/1 When my mum was at school, they wouldn't let the boys wear
patent-leather shoes because they thought they would use them to look
up girls' skirts. I guess this was the beta version of upskirting.

2017 Times (Nexis) 15 Aug. (Times 2 section) 7 The offence of
voyeurism doesn't cover upskirting.

The term upskirt appears to have been used as early as in June 1998:

“A video, usually taken in a crowded location such as a shopping mall, that is shot up a woman’s skirt” without the woman’s permission or even knowledge. (Source: Word Spy)

Urban Dictionary’s earliest entry for upskirting is from 2006, but Word Spy found a 1998 mention in the Washington Post:

• What began as a small photo gallery on the Internet a couple of years ago has rapidly expanded to more than 40 such “Upskirt” sites, including one devoted entirely to shots taken up skirts in Maryland, said [Alexandria detective Harold] Duquette, who has been tracking the trend.
  —Patricia Davis, “Video Peeping Toms Seeing More Trouble,” The Washington Post, June 7, 1998
  

(nancyfriedman.typepad.com)

upskirt (adj.) Etymonine.com

by 1997, from up (adv.) + skirt (n.). As a verb by 2008.

"Upskirt" videos, usually taken using low-hanging bags, feature
up-close-and-personal crotch shots of leggy, panty-clad young women.
["Weekly World News," Sept. 29, 1998]

The word is ~ 20 years old. The practice? Since Adam and Eve were evicted from the garden! Small cameras etc (digital) have facilitated such. I suspect though the practice of 'photographing' occurred back in the 8mm and flash photography days.

The earliest use I can find of upskirt in a related sense is in Jack van Niftrik's 1980 novel Where Rumour Never Reaches:

He experienced the contrasting sensations of sorrow at the plight of the Nongamas, the Bikos and the Molefes of the world and a flash of sexual desire by a breathtaking upskirt glimpse of honeyed thighs and white triangle of underwear.

This use is of course an adjective rather than a verb, and it seems that adjective and noun uses are by far the most common. The OED only lists adjective and noun senses, though it does list the noun upskirting as a separate noun (earliest attested use being in 1998) which it says is derived from the adjective rather than from upskirt as a verb. (This does not mean it is denying a verb use that it doesn't record).

Of course, English speakers verb adjectives and nouns all the time, whether directly or to form a gerund form like upskirting, so as long as we can find the adjective upskirt as something deliberately sought (rather than the accidental glimpse van Niftrik's novel describes) we can expect it. The OED has both adjective and noun forms of upskirt attested in 1994.

To conjecture about how far back it may go we can ask how far back we could reasonably expect to find upskirting. Pornographic pictures showing a view up a skirt exist from the early days of photography, (and in painting Fragonad's Les Hasards heureux de l'escarpolette shows a man enjoying such a view, without sharing it with the painting's audience, as far back as ca 1767 while Marilyn Monroe's famous revealing moment in The Seven Year Itch in 1955 was not the earliest in moving pictures by a large measure) but since the gerund form generally refers to deliberately surreptitious pictures taken without consent, the phenomenon of upskirting really depended reasonably fast film and either on the telephoto lens, or small "spy" cameras, so really not much earlier than the 1980s as something that wouldn't need remarkably exotic equipment. (Ubiquitous mobile phone cameras of course make it much easier today).

One only needs terms when discussing something though, which means not just someone actively taking such photographs, but a community of people sharing them and discussing the taking of them. That doesn't really seem to have predated Usenet in any large-scale and continuous manner, so we could take 1980, being the time that it first became possible to share binary files such as photographs on Usenet, as the earliest it could possibly go back to. Realistically we're probably talking about the late 80s or early 90s, but almost certainly no earlier than 1980, and clearly by the time of the first OED attested use in 1998.