Hcfyk

I don't understand why we use "unattended to" instead of "unattended" in these examples.

1-Is a gunman 20 years-old simply a troubled youth who got overlooked
and unattended to?

2-Shell, says the report, claims to be a good neighbour, but leaves
oil spills unattended to.

3-Please do not leave your luggage unattended.

Unattended is the complement of attended, and by the same token unattended to is the complement of attended to. To attend to something means to give it attention, to look after it, resolve it, clear it up, etc. as appropriate.

Thus, something that is not attended - does not have someone with it - is unattended, while something that has not been attended to is unattended to.

The "troubled youth", the question ponders, might simply not have been given the care that would have helped him. Shell doesn't clean up their oil spills, one quote alleges. The last one simply asks you to keep your luggage with you.

If you look at https://www.merriam-webster.com/dictionary/attend , you will see a list of definitions under "transitive" and another list under "intransitive". "transitive" means it takes a direct object, while "intransitive" means that any object will be indirect, i.e. will have a a preposition. In the case of "attend", that preposition is usually "to". One of the intransitive definitions is "to be present with : ACCOMPANY", which is being used in the luggage case. The intransitive sense has definitions such as "to direct one's attention". There is also a meaning of "deal with" that isn't listed by this dictionary. These meanings fit the other cases you cite where "to" is used.

The adjective unattended has primarily two meanings:

1. Not noticed or paid attention to, or dealt with may be (yet is not quite often)*, followed by the preposition to:

Is a gunman 20 years-old simply a troubled youth who got overlooked
and unattended to?

(Wasn't there anyone to attend [to notice or pay attention to] to the youth?)

Shell, says the report, claims to be a good neighbor but leaves oil
spills unattended to.

(Shell claims that she is a good neighbor even if she doesn't attend [doesn't deal/take action with respect to] to the oil spills she leaves.)

2. Not supervised or looked after (doesn't take the preposition to):

Please do not leave your luggage unattended

(Don't leave your luggage without your [or someone else's] supervision.)

The sourse with examples of the usage.

*In my opinion, the difference between the two meanings is very often too subtle in many scenarios.

Examples 1 & 2 are incorrect uses of the intransitive verb "attend to"
since both end sentences with a preposition; "to" in these instances.
These are sentences that ought not to be written as they are.
"Not given attention" would be a preferable construction in example 1 and "unremediated" would be preferable in example 2.

Example 3 is a correct use of "unattended" as an adverb, the verb is "leave" in this instance.

TL;DR Examples 1 & 2 are incorrect grammar which makes them difficult to understand. Two reasons;"unattended to" isn't the negation of "attended to", & sentences shouldn't end with "to".
The difference in meaning between examples 1 & 2 and 3 is due to 1 & 2 using "unattended to" as a verb and 3 using "unattended" as an adverb.