As a dev myself, here's a personal take: this is dangerous at scale.

Workshop likely has some measure of moderation baked in so that UGC submitted to be allowed as items through Steam can be yeeted by people with the right access level (either VALVe filter team or the developers). However, custom game art, by definition, doesn't need that because "it's your choice of art".

The issue here is, this would still be considered User-Generated Content hosted by VALVe, at least in the eyes of the law (IANAL - this is an assumption). An easy issue that arises is when certain types of, to put it politely, extremely distasteful and illegal media, are used as custom art for any element, and get synchronized to the VALVe cloud. At that point, VALVe is on the hook for hosting the media, and in the eyes of the law (IANAL - this is an assumption), might become a "willing participant in the propagation of illegal media" simply because they facilitated synchronizing "custom art" between one user's machines.

This is a dangerous feature to implement. I know I have no say in this and no pull whatsoever, but I would recommend against even considering implementing this.

Thanks for your comments; interesting take.

Do note this Feature Request has 2 parts:

1. custom image sync via cloud.
2. custom-game-identifier/sync-key for Non-Steam games.

For (1.), I'd say let Steam staff determine if it is something feasible/desirable.

For (2.), I'd argue it is a worthwhile feature, because even if (1.) never came to fruition, it would still allow people to sync their custom art by other means, with predictable results.

I'm not saying it's not a worthwhile or desirable feature. What I'm saying is, it's a dangerous one to implement, let alone investigate.

As a dev myself, here's a personal take: this is dangerous at scale.

Workshop likely has some measure of moderation baked in so that UGC submitted to be allowed as items through Steam can be yeeted by people with the right access level (either VALVe filter team or the developers). However, custom game art, by definition, doesn't need that because "it's your choice of art".

The issue here is, this would still be considered User-Generated Content hosted by VALVe, at least in the eyes of the law (IANAL - this is an assumption). An easy issue that arises is when certain types of, to put it politely, extremely distasteful and illegal media, are used as custom art for any element, and get synchronized to the VALVe cloud. At that point, VALVe is on the hook for hosting the media, and in the eyes of the law (IANAL - this is an assumption), might become a "willing participant in the propagation of illegal media" simply because they facilitated synchronizing "custom art" between one user's machines.

This is a dangerous feature to implement. I know I have no say in this and no pull whatsoever, but I would recommend against even considering implementing this.

ok, i'm a little lost on this one

if this would be a problem for steam in this context, how do other online services that synchronize and store user-submitted data (cloud storage and file sharing, most online games, imageboards and media sites, and the entire rest of steam itself) not have this problem?

if this was a real concern here, the fact that it's trivially easy to put any files into steam cloud by adding them to a game's save data would render it relevant to the entirety of steam cloud, right?

ok, let me address some specific points you brought up:

Workshop likely has some measure of moderation baked in so that UGC submitted to be allowed as items through Steam can be yeeted by people with the right access level (either VALVe filter team or the developers). However, custom game art, by definition, doesn't need that because "it's your choice of art".

likely? that sounds a little uncertain
the steam workshop is almost entirely public, so of course it has moderation
newly submitted items are checked by valve before they can even be seen publically, as is good practice
and, if this problem existed here, valve could just do the same for custom game art
or, if it doesn't need it, it doesn't need it
valve would have a pretty good idea of that

At that point, VALVe is on the hook for hosting the media, and in the eyes of the law (IANAL - this is an assumption), might become a "willing participant in the propagation of illegal media"

this varies between jurisdictions, and i'm no expert on any of them, but if this were a problem here, it would again apply to all of the above, right?
also, you seem to be making a lot of assumptions

I'm not saying it's not a worthwhile or desirable feature. What I'm saying is, it's a dangerous one to implement, let alone investigate.

let alone investigate?
there's nothing wrong with investigating if something should be done, which you would usually do before implementing anything
also, you can stop calling it dangerous now, there's nothing dangerous about part (2.), and valve will know if (1.) is

i'd like to make it clear that i'm not trying to insult or upset you, and i see where you're coming from, i just don't think your advice here is the most well thought out, and it reads as a little assertive for how unsure you are of some important points

this might also be useful for keeping other properties of games in sync, such as collections, favorite status, remote play, recency, playtime (currently not tracked for non-steam games), etc

I'd personally love to see this feature implemented as well, but even ~3 years and two months later, I still think UGC is a somewhat dangerous terrain to "play" in if they don't have solid enough a legal definition of what is theirs and what isn't.

and it reads as a little assertive for how unsure you are of some important points

You're not wrong. On one side I'm trying to say "I think there's grounds to believe they have good reasons to not do this", on the other I'm also saying "I have no idea if what I'm saying is anything even REMOTELY close to real". My deepest apologies for the confusion.