What games have been on your wishlist the longest?

Split/Second since February 2018

Nitroneers on 03/09/2019 (never released game ...)
Relic Hunters Legend on 06/11/2019
Electro Ride: The Neon Racing on 15/11/2019

Started Steam on 10 september 2017.

My oldest is Tunic, since 2018. It was relased in 2022.
Shovel Knight: Teasure Trove - 2018
Rehtona - 2019

Some games remain on my wishlist for a long time before I buy them, mainly due to getting the time to play the any similar ones I already have first.

2016-04-19 :: Vampire: The Masquerade - Bloodlines
2016-04-19 :: E.Y.E: Divine Cybermancy
2017-10-07 :: The Elder Scrolls® Online

Not sure about TESO but I definitely have to give the first two a try... someday

Lamplight City, 13 sept 2018.
I believe it was in my WL even before that, I must have taken it off and put it back on at some point.

And now I found out I'm waiting for N1RV Ann-A: A Cyberpunk Bartender Action since 2019 :')

Heh, just cleaned up my wishlist a week ago, by quite a lot too.

Atomic Heart - 5/18/2019
G String - 10/17/2020
Lost in Vivo - 1/12/2021

I keep expecting Atomic Heart to show up in a Humble Choice (publisher is Focus Entertainment, who are no stranger to Humble Bundle), otherwise I would have bought it by now.

Valiant Hearts: The Great War - 2014
Secrets of Grindea - 2015
Gravity Ghost - 2016

It looks like my oldest are

1. Flashback (2013-10-02)
2. Grand Theft Auto V (2015-01-16)
3. Divinity: Original Sin - Enhanced Edition (2016-01-02)

Sekiro™: Shadows Die Twice - GOTY Edition on my Steam wishlist since 2018 but I don't really use the Steam wishlist for wishlisting, I use it mostly to make GAs stand out for some wishlisted games and to track down unreleased games.

I used ITAD in the past but I moved most of my wishlist to GG recently so I can't answer for sure

Haven't been using Steam, well pretty much not at all, until lately, so my 3 oldest appear to be:

1. Baldur's Gate 3 - added on 01/03/2020
2. Nivalis - added on 13/06/2023
3. Shadow Gambit: The Cursed Crew - added on 26/06/2023

My oldest 3 are from 2015:

1. Zeit²
2. Gran Vitreous
3. Catacomb Kids

Gran Vitreous was in one bundle (back when you could get 16 games for 2.99 in a bundle)... Other 2 have never been bundled as far as I know.

Dying Light Added on 7/9/2013

August 2016
Hyper Light Drifter
Dust: An Elysian Tail
Shadow Complex Remastered

I used to regularly delete my entire wishlist, but i got seriously ill at one point and haven't done it since

anyway my 3 are
Shahrzad - The Storyteller from August 2018
Archaica: The Path of Light
Pavillion

Going back through the wishlist is kinda interesting as there were some games which I have no memory of adding or no longer appeal

The oldest ones (still in the list):

1. The Universim - 12/2015
2. Mount & Blade II: Bannerlord - 10/2016
3. Two Point Hospital - 01/2018

My three oldest are Towerfall ascension, catlateral damage, and duck game. All from 2014-2015. Forgot these were still on here, might have to clean them up during sale.

Game not added on Steam
Assassin's creed Origins

Games added on Steam
Outer Wilds
Kingdom Come Deliverance

The forest is followed by dark messiah and D4, all in early 2015.

According to a quick sort by date added, it is From Dust, which makes sense - it's in a genre I like, but the mediocre reviews have always deterred me from spending money on it; and since it's an old game whose publisher doesn't do much with it, I never randomly acquired it in a bundle. I should probably just remove it - it's 75% off right now and I still don't feel the urge to buy it.

Let's see:
Melody's Escape since 2014... which predates its release date so I guess that one had early access? I guess the fact that I still haven't bought it, in key or Steam Sale form, shows that I kinda don't particularly care about it, but the fact that I haven't removed it during one of my Wishlist cleanups shows that it's too cute-looking for me to ditch it forever. I think it was going on a decent sale the last few major sales...

There Came an Echo since 2015 - the not-really-sequel to Sequence (which was later renamed to Before the Echo)... eeeeeehhhhh.

Flywrench since 2015 - nice-looking arcade game which... wait a second... checks its achievements... oh, which has a bogus achievement... and I don't mean "difficulty bogus", I mean stupid online gimmick bogus... I know which game will be on the chopping block next time I get too close to 100 wishlisted games... <.<

I think my oldest was Axiom Verge, but I have that in my library recently. My top three are:

Flint Hook - 11/22/2017
Cuphead - 11/22/2017
Dead Cells - 12/3/2017

Resident Evil 6, added in early 2013 before it even released. I think I finally bought it in a bundle recently, but never activated the key because, well, it's Resident Evil 6. Can't bring myself to remove it though.

Oldest DLC
XCOM 2: War of the Chosen - 7/14/2017
Oldest Games
FINAL FANTASY XII THE ZODIAC AGE - 2/2/2018
Impossible Creatures - 6/19/2018
Graveyard Keeper - 8/25/2018

The Binding of Isaac: Rebirth
Added 11/21/2014

Game released over 10 years go. Lowest recorded price in USD: $5.09

Xenonauts
Added 5/7/2015

Game released over 10 years go. Lowest recorded price: $4.99

Zero Escape: The Nonary Games in June, 2020.

Not so long to be honest :)