how do or where do people keep their steam keys?

just notepad with a password set .
and if its just for the mystery bundle you should be fine with a notepad .
just retrieve the keys from the notepad instead of the fanatical site .

you can just make 2 rows, used|not used , and then move the key in the "used" after u give it away ¯_(ツ)_/¯
This said would be better if write it in Paper Book instead of PC,pc password is nothing if hacker hack gpu\vram can see screen all u write (comprised password),instead paper book (if you is not put webcam\mic or something) is can't see,just use can delete pencil insteand of pen

and if make 2 copy is better,you can never know what can happen in life (bad events),if put all only 1 paper\notepad then if for whatever reason the paper\notepad will lose\destroy is lose all keys\data too so better make 2 copy different places.

I just use an excel sheet ,colour the keys I have sent and delete them when are marked as received

Text document, notepad.

I have an excel spreadsheet where I keep my keys with some basic information for me - name of the game, the key itself, state (e.g. some keys I know that they might be duplicate), and source of the key (in case there is some problem, it saves me a lot of time).
It's pretty nice using excel because I can easily sort the keys by alphabet and easily copy all of them for sites like steamtrades.com.

I store them on Google Drive.

I have way too many unused keys from past bundles, it's too dangerous to store them on local drive in notepad.

MS Access database. One column for the key, and another for the status.

game_keys.txt
but I edit it with notepad++

I just use them within the same week either for myself, for friends or for Steamgifts so I don't store anything.

used to be sql database, now it's just a text file

In Swiss banks

notepad and sometimes a bookmarked page if I rarely shop there and I'd forget it

Games for myself I activate instantly. Games I make giveaways for I make the giveaway instantly. I rarely do trading but when I do I save them in Evernote.

I used to keep them all in a txt document but now I have them all over the place : /

I don't store any keys either I use them for myself or make giveaways. Each digital store has somewhere in their small print a clause telling you they only guarantee your keys are working for a limited time e.g. 6 months.

Excel. In an encrypted drive.

multiple tabs, one for each platform.
The steam tab contains most things however.

Col include color coding for sets and bundles, code/copy, no cv date, acquired type, acquired date, costs, retail, store, name , notes, url, used status, redeam/giveway, used date, winner/creator, other IDs,

Generally "notes" will have the key or say "onsite" if I have not reveled it., The key gets moved to the ""used keys" col once used,

I have scripts I wrote the fill out various col depending on what is col is active when I hit a hotkey.

i put mine in a box and bury them in my back yard.

notepad, delete key when posted

redeemed

notepad, dont understand why anyone bother with excel, making colorful charts, tables etc... i see it as pointless time waste
code - game name ... nothing else needed at all. Works like a charm...

I keep a list on barter of what keys I have that are unused, but the keys themselves are mostly still unrevealed.
Will sometimes put them in a txt file if it's more convenient though.

I keep unredeemed steam keys in Standard Notes, I usually store them there when I want to gift a certain individual on a special occasion.

The reason I choose Standard Notes is that it's completely free to register, it's secure, it syncs through all devices which I find very helpful. So, I personally recommend it to any note-takers.

View attached image.

I use notepad file which I update each time I trade( or gift) a game, but the funny part is that I upload it to facebook massage and delete it afterwards and do that each time xD I know it's not really practical but it just became routine.

I memorize them.

its kind of a complicated system

View attached image.