Your best option is to not use premade assets. It's always best to learn to do it yourself, especially because of the limiting restrictions in question. On top of that even if an asset doesn't have restrictions that could change at any point in the time the creator decides to change it.
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Well placeholders are just that, they're meant to be replaced. I am specifically talking about an official release of any game, hell even YouTubers would get screwed over for using music in videos that was "free" and later had it's terms and conditions changed. I have a lot of points against using premade assets but it becomes a whole discussion if I bring them up.
Sorry, allow me to be clarify my previous comment. I am agreeing with you. Pre-made assets are designed to be placeholders and should be replaced before the final release of the project if the developer can afford custom art or can make the art themselves.
Small caveat to cover my butt and is not necessarily in response to any previous comment: Many developers on itch.io are unable to commision art or create art themselves. There is nothing wrong with indie game devs using pre-made assets in the final versions of their projects so long as they are used as the artist intended. As an artist, I love seeing the art I sell on itch.io in people's games because it's like giving life to something I created.
I didn't assume you were agreeing or disagreeing, I mostly thought you were confused at what I claimed. However on your second point, no one is "unable" to create their own assets. It's all about time, willingness to learn, and in a way 'stubbornness' but really just not giving up. Like I said I have a lot points about this topic but unless you want to read an essay I will hold them back.