Wizards gave us the top sixteen deck lists for the three Eternal Weekend tournaments held on Magic Online this year. While more data would be lovely, here is a comparison of the five Doomsday lists that made the top 16 in all five tournaments. Worth noting is that four of them were Top 8, one Top 16 and one the winner.

First, let's take a look at the lists.

Vintage means lots of one-of, and with a combo deck like Doomsday we tend to have a fairly solidified core. Let's take a look at this "core" of the main decks. That is, any card that was at least a one-of in all of them.

Fifty-one cards are core to these decks, giving about nine flex slots. Fetches were grouped together and Snow or not doesn't matter for Doomsday. It is worth noting Thiim ran one Verdant Catacombs but interestingly it was Gerschi with the Snow-Covered Swamp. Thiim did have a Swamp in their sideboard, but we'll get to those later. Thiim's list throws this off a bit because of their running of 12 lands main.
Anyway, let's look at these nine flex slots.

If we think of Discard, Bounce and Counter as forms of interaction we see almost everyone with 4 pieces of interaction. Lumping Search with Draw would give us a about 2 or 3 except for Gerschi, who was on an extra piece of interaction and mana. Thiim's deck seems to be the most different from the rest.

Sideboarding comes down a lot to personal preference and what the expected meta is. That said, in Vintage it is fair to assume Bazaar, Shops, Xerox, DRS, and Combo matchups. (Wasteland/Hatebears might also be a solid consideration as well.) Here's how I'd consider each piece.

Bazaar

Gurmag Angler, Tasigur, the Golden Fang, Bloodchief's Thirst, Feed the Swarm, Fatal Push, Surgical Extraction, Leyline of the Void, The Tabernacle at Pendrell Vale

Shops

Gurmag Angler, Tasigur, the Golden Fang, Bloodchief's Thirst, Duress, Feed the Swarm, Eliminate, Hurkyl's Recall, Steel Sabotage, Island, Karakas, Swamp

Xerox

Bloodchief's Thirst, Duress, Feed the Swarm, Eliminate, Flusterstorm, Mindbreak Trap, Mystical Dispute, Fatal Push, Cavern of Souls, Karakas

DRS

Gurmag Angler, Tasigur, the Golden Fang, Bloodchief's Thirst, Duress, Feed the Swarm, Eliminate, Flusterstorm, Mystical Dispute, Fatal Push, Cavern of Souls, Island, Karakas, Swamp

Combo

Duress, Feed the Swarm, Flusterstorm, Mindbreak Trap, Mystical Dispute, Surgical Extraction, Cavern of Souls

This is obviously a contestable part of the data, entirely based on what you count as what for what. That's a sentence. Still, it seems even using these standards most players had about the same range of cards. Plenty for Bazaar and Shops with a fair amount of cards for the rest, making sure to do double (or triple) duty where possible.