Easy way to fetch appIDs?

I am not entirely sure, but my guess is that nowadays Steam has a "new" collection database (since the library overhaul), and an "old" one. Depressurizer only seems to handle the old one for now... But there is a way to replace the data in the new one with the data in the old one (if Steam is running, in Windows, do "Run" and "steam://resetcollections" -- careful, this really REPLACES your collections with the old ones).

So, the trick is, if you exclusively use Depressurizer to make your collections, then import into Steam (well, every time you change something), then both "old" and "new" one will be synced, and then you can do everything through Depressurizer. Hm, it sounds like a lot of work the way I described it, but it is actually quite convenient once you get it going.

Of course, I am not sure if this helps you, since I do not know how much you would have to redo things with Depressurizer from older data to get to your new collections, or if they are new at all. And you would still have to figure out how to get the Appids from Depressurizer's JSON data, but this sounds easy.

Hm, okay. Not sure if this has been changed some time ago.

Well, with a list of all appIDs, you can check them against SteamDB with the usual URL (https://steamdb.info/app/<ID</ )..
Could be done with a simple shell script or something, just one loop and the usual curl & grep..

But there might be a better alternative:

Simply access the Steam Web API in your browser, this should always work if you use your own Wep API key together with the matching User ID. Even if the profile is set to private, I think.

The correct API endpoint is this:

https://api.steampowered.com/IPlayerService/GetOwnedGames/v0001/?key=xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx&steamid=yyyyyyyyyyyyyyyyy&format=json&include_appinfo=true

Where key=xxxx... is your Web API key, which you can simply view here: https://steamcommunity.com/dev/apikey
Where steamid=yyyy.. is your steamID64 user ID, which you can get from many places, here e.g. https://steamrep.com/

(The steamid number is the numeric part behind https://steamcommunity.com/profiles/) (Usually starts with "7", I think, maybe not for newer accounts)

You should get a JSON-formatted output of all your games on your account (Format can be changed with the $format=option).

Simply copy that into a file or something, and then process to your own liking..

A simple pretty-printed overview can be created like this, for example: jq '.response.games[]' INPUTFILE.json

(You can test different options/queries with jq here: https://jqplay.org/)