Legal status of voluntary homelessness among tourists/short term visitors

There are actually two intertwingled questions here:

First, virtually every country requires that (if requested) you demonstrate the ability to support yourself for the duration of your stay. For example, if you fly to the US and then declare at immigration that you only have $10 and intend to sleep on the streets for the next two months, you will certainly be sent packing -- since, regardless of whether doing so is a crime, it would almost certainly incur costs to deport you later.

Second, once you're in the country, with few exceptions, laws do not discriminate by nationality: when in Rome, you must do as in Rome, whether you're a citizen or not. So if it's illegal to do something in a given locality (sleep in a car, sleep outside, "vagrancy", "loitering"), it will be illegal both for visitors and for citizens.

Once arrested, if they find out that you're not a resident, whether deportation proceedings are commenced will depend on the severity of the crime, the policy of the jurisdiction, whether you have any money left or have become a "public charge", whether you've exceeded your period of stay, etc.

For more specific answers:

Say I am a Canadian Citizen and fly to the U.S. and, skipping all the details, I'm in the country and then decide to voluntarily sleep in the streets. This is generally NOT illegal for citizens/residents themselves, but is it if a visitor would do it? Not assuming they overstay their six month period welcome -- just that they are legal visitors in the country who chose to NOT reside in any accommodation like hotels/homes/temporary living facilities/homesharing/etc. Is it illegal or it makes no difference?

It makes no difference.

This is assuming they are just the same as any other homeless (e.g. not committing any crimes or disturbing others) -- only that they're not permanent residents. I mean I don't think police would deport you so immediately for just being in the street itself.

Only if it were a crime to be on the street in the first place ... then maybe.

And would it make a difference if the person has the option to live on the streets or otherwise?

As the other answerer said, as long as you make it clear that you can afford your stay somehow or with some arrangement you will have no issues. To make things even easier, immigration officers tend to go by your WORD only since I've never heard of one demanding bank statements or forcing you to show them your bank account balance in front of them on the internet.

Once inside, you are bound by laws as everyone else -- so if it's not against the law to live in the wilderness or behind a Taco Bell, it won't be a difference whether you're a visitor or citizen.

So if one wanted to live on the streets/in the woods upon entry, the solution is simple: lie. That's it. Keep yourself from doing something in which you could be charged with a crime. Or have a really good reason for living in such an arrangement that you're sure can't be denied by an officer.