TLDR: This is a list (incomplete) of companies that advertise with Reddit and put money in its pockets, and some contact forms for popular companies.
Brilliant Earth (www.brilliantearth.com - scroll to bottom and choose contact method)
Universal Studios (https://www.universalstudios.com/contact/general-and-miscellaneous)
Microsoft
Focus Features
Mitsubishi Motors (https://www.mitsubishicars.com/contact-us)
IBM
Intuit
Autodesk
Electronic Arts
Ally Financial
Discover
Up Australia/Up Banking
ClearScore
Noosa Yoghurt (https://noosayoghurt.com/contact/)
BMW Mini Cooper
Adobe (https://www.facebook.com/Adobe/)
Adidas (https://www.facebook.com/adidas/)
Adrenaline Australia
GameStop ([email protected])
H&M (https://www.facebook.com/hm OR https://www.instagram.com/hm/)
Liquid I.V.
Oatly (https://www.oatly.com/contact OR https://www.facebook.com/Oatly/ OR https://www.instagram.com/oatly/)
JOE & THE JUICE (https://www.joejuice.com/contact)
Excedrin (https://www.excedrin.com/contact-us/)
Rayovac (https://www.rayovac.com/contact-us/)
Nespresso (https://www.facebook.com/nespresso/ OR https://www.instagram.com/nespresso/)
Novo Nordisk
BackMarket
Caliber Fitness ([email protected] OR https://caliberstrong.com/contact-us/)
Lucozade (https://www.lucozade.com/support/)
Moen
Uber
HP
Tezos
Truebill
Ulta Beauty (https://www.ulta.com/guestservices/contact-us)
MeUndies (https://meundies.kustomer.help/contact/how-can-we-help-HyfOyZn0o)
Lagunitas
Aviva
Beyond Meat (https://www.beyondmeat.com/en-US/contact/)
Bitstamp ([email protected])
Hootsuite
Zoetus
Wolt (https://explore.wolt.com/contact)
Fineco Bank
Alienware
Tails.com (https://shop.tails.com/contact-us/)
Duracell (https://www.duracell.com/en-us/contact/)
Creative Assembly/Total War: Warhammer III (https://www.totalwar.com/support/)
Finder Australia
Virgin Galactic
Bungie
Discover Financial Services
Capcom
Allergan Aesthetics Coolsculpting
So what would happen if I sent a thoughtful message about how disappointed I am with the advertiser for sticking with Reddit through these changes. What if I discussed how what’s going on there is a breach of trust against its own content creators and resource providers (us). And that they are devaluing their own product (us & insert advertiser).
Would that be worth anything?
I personally don't think that a message like that would make much of a difference because they won't understand the nuances of the situation that's occurring. All they care about is eyeballs on their product, and avoiding association with unsavory things.
Because of that, I would think that a better tactic would be to talk about how you saw their advertisements in a subreddit with inappropriate content and ask if that was intentional. Asking if it was intentional is a good tactic because it turns it from a report into a question. Even better, word it like "Do you have any comment?" as if you're a reporter writing a story. You don't have to say who you're writing "a story" for, you can just say that you're an independent journalist working on a piece and wanted to get a statement on their choice to place advertising on a service that serves their advertisement next to (give some examples)
With that tactic, you can also skip the standard feedback forms and attempt to contact their advertising arm or PR department.