Steam store page lists trading cards as feature, but no drops:
The game is still affected by low metric confidence. That's something Valve introduced in June 2018 to restrict bad games, asset flips and so on from being displayed in profiles, dropping cards, displaying achievements on profile pages.. Games which have been released afterwards and haven't reached the metric yet, show as "Steam is still learning about this game" or "Profile Features Limited", games released before that unfortunately doesn't list it (e.g. https://store.steampowered.com/app/745670/3D_Tower/). What is the metric? Store purchases (not key activations), reviews, maybe even a certain threshold of playtime, only Valve knows the exact metric.
Now some games list "Trading Cards" as feature, because the developer added the cards (the artworks) to the game management page. Due to the metric not reached, drops aren't enabled though. The dev/publisher can't get around it, I assume he could only remove them again, so there isn't any "false advertising". This might lead to what you described as "available one day and disappear next".
Steam Card Exchange not showing some cards:
Steam Card Exchange is hosted and maintained by a small team, not affiliated with Steam/Valve. Their weekly report shows which games were recently added (either new unlimited games, from top publishers e.g., or games for which restrictions were lifted and have trading cards added by the dev). They probably crawl the artworks with a bot to integrate them to their showcase, but can't estimate the rarity without data at this time. They also can't display prices, if there haven't been any offers at the market yet. If games or other information are missing, you can contribute within their Steam group.
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Thanks for the detailed response. Makes a lot more sense now.
I've also noticed some games I've played quite a number of hours, suddenly drop 4 or 5 cards after a few weeks since I've played them.
I do a lot of card selling (for my Steam Wallet to afford higher value games) and sometimes badge building, so I'm always on the lookout for games with cards here and there.
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Some games didn't have cards at launch but then the developers decided some time after release (this can be months or years later) to add them. If you have already played enough hours to warrant card drops you are getting them all at once.
This is getting more and more rare though, these days most either come with cards from the start or will never get them.
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FYI regarding the "crawl the artworks with a bot to integrate them to their showcase".
The data SCE now relies on is pulled from an API and from the Points Shop. As Backgrounds and Emotes are now available through the Points Store it is easier to find ... though as pointed out the rarity still needs data, since the Point Shop doesn't provide that detail. This process has made managing the site a lot easier for GD (GermanDarknes) who before had to input based on data given by members (use to post the background URL, etc).
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The Steam store tags like trading cards are usually set by the publisher/developer of the game and not by valve. Sometimes these tags are just wrong. E.g. the game claims to have trading cards while the game doesn't have them or isn't allowed to drop cards (steam is leaning about games) or don't show trading cards while it actually has them.
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I'm curious about the Steam Trading Cards. For example, when I view a specific game on Steam, it indicates that there aren't any Trading Cards, but when I view Steam Card Exchange, it indicates that there are cards available.
Additionally, it often changes when viewed on Steam from time to time, while also very rarely there are incidents whereby cards are listed, but can't be found on the Steam Card Exchange.
I'm not quite sure which to believe in the end and why they would be available one day and disappear the next. Any thoughts?
GA for your curiosity.
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