I know the description says it has to be a Steam key that I own.
But I wanted to know if it's still possible to provide a non-Steam key via Giveaway as long as it is clarified in the description. All this with the intention that someone who wants to can take advantage of the game and not get lost.
It really depends on what kind of key you're trying to give away.
If it's a key that redeems on another platform (e.g. GOG.com) then you won't receive much flak for it if it's a valid code. You could still receive a negative feedback if it's not a Steam key if the winner wanted a Steam key, though.
Do not use any of the following keys to start giveaways here, because you
WILL catch flak for it:
1. Demo/Trial/Playtesting/Alpha/Beta keys. (This is strictly against IG giveaway rules)
2. In-game redemption codes (that redeem within a game's store/rewards center) Please give those away
here.
3. Keys for things that are already free to obtain on Steam/GOG/WhereverElse. (Steam is okay if it's a game key for a game with card drops)
4.
DO NOT post keys that do not match the label that you choose when starting the giveaway (eg. starting a giveaway marked as a full game, but putting in a DLC key instead)
5.
DO NOT post keys for expensive games/DLCs that are obviously only obtained via direct purchase from Publishers/Steam. (eg.
this or
this)