I'm going to provide a dissenting opinion here. I think the URL is for location, not state. I believe that using the URL as a state container leads to unexpected and unwanted behaviour.

First, I think it's a fact that the average user does not consider a URL to be a state container. The fact that developers in this thread lament the "new school" React developers who don't use the URL as a state container is proof of this. If it follows that a React developer, no matter how inexperienced, is at least as knowledgeable if not more about URLs than the average person, if they don't even consider the URL to be a valid container for state than neither does the average person.

Putting state in the URL breaks a fundamental expectation of the user that refreshing a page resets its state. If I put a page into an unwanted state, or god forbid there is a bug that places it in an impossible state, I expect a refresh of the page to reset the state back. Putting state in the URL violates this principle.

Secondly, putting state in a URL breaks the expectation of the user for sharing locations. When I receive Youtube links from friends, half of the time the "t" parameter is set to somewhere in the video and I don't know if my friend explicitly wanted to provide a timestamp. The general user has no idea what ?t=294833289 means in a URL. It would be better to store that state somewhere else and have the user explicitly create a link a timestamp parameter if the desired outcome was to link to an explicit point in the video. As it stands now, when I send YouTube links to friends I have to remember to clear the ?=t parameter before sharing. This is not good UX.

There are other reasons why I think its a bad idea but I don't want this comment to be too long.

That doesn't mean not to use search parameters though. Consider a page for a t-shirt, with options for color and size. This is a valid use case for putting the color and size in the URL because it's a location property - the resource for a blue XL shirt is different from a red SM shirt, and that should be reflected in the URL.

That's not to say that state should never be put in the URL - in some cases it makes sense. But that's a judgement call that the developer should make by considering what behaviour the user expects, and how the link will most likely be used. For a trivial example, it's unlikely that a user wants to share their scroll position or if a dropdown is open when sharing a page. But they probably want to share the location they've navigated to on a map, as it's unlikely they're sharing a link to `maps.google.com` with others (although debatably that's not state, but rather a location property).

I strongly agree with this, just couldn't be bothered to type it out. I've tried it both ways many times, and you are indeed right on the money.