Steam library: Can games be hidden by removing them?

Ok, firstly, no calling out. You don't need to go into specifics about something to the extent everyone can easily tell who you're talking about.

Secondly, yes, this could be done, but they would have to have removed the game (and re-synced) before entering for your game giveaway.

Thirdly - just cos they have a ton of games from that franchise doesn't mean they would do something like that - it's not like you're giving them a key they can resell.

The removal part is presumably doable, however SG tracks the games a user has on profile. So unless it's a "Beware I have this game and still can enter" title, it's not very straightforward.

Only if they bought / activated it within the last 7 days and then removed it before entering your giveaway, I think. Even with a private profile, you have to sync your games with SteamGifts at least every 7 days. So SteamGifts should know, if they ever owned the game before that time frame. So while I don't know for sure, it would be a serious and avoidable loophole if SteamGifts didn't check that.

Also: that franchise is huge. SteamDB shows at least <MANY> different games (yes, I left out DLCs) with "<FRANCHISE>" in the title:

https://steamdb.info/search/?a=app&q=<FRANCHISE>&type=1&category=0

Owning <SOME> of these is not that much in this case.

Edited to remove the actual franchise name and number of games

...can that user remove the game he or she won from the Steam Library and then, after receiving the key from you, undo the deletion of that game?

Yes

And it give some people that use this regular.....
... but it is very hard to have enough evidences for this.

As far as i know from my own experiences it the only way, that the mods accept, that you check the profile for achievements in game XYZ. Because the badges don't get removed when you delete a game. Of course it could be done with SAM but cheaters are in the majority dumb people, so you catch a lot of them with this checks, but the checks are time intensive and nothing you can do for each game. And the game shouldn't have a Demo that drops achievements or the checking method isn't possible.

I send a reroll request for each winner with a private profile (standard). The mods can check private profiles for unactivated wins.
First when i have their "all ok" i send the key.
That is better as nothing because it catch from time to time a cheater that try to hide his not activated wins.

Fuck private users, why even bother joining giveaways.
Their just some scummy cunts to blacklist