Is a person sinful for leaving a discarded can in the street that says "peace" (السلام), if what is meant by it is safety and security, and not the name of Allah; or for leaving names like "Ali" and "Omar"? And what is meant by letters having sanctity?

Nothing besides the names of Allah, the prophets, and the angels is obligatory to respect. However, it is recommended to respect them by not throwing them on the ground, due to the honor of the letters. Neglecting this is contrary to what is preferable. If it is difficult to do so, there is no harm. Ibn al-Hajj mentioned the reprehensibility of disrespecting the names of scholars and righteous people, and that it does not reach the level of prohibition. The intended meaning of the حرمة (sanctity/sacredness) of the letters is their honor, and the relied-upon opinion is that this sanctity applies to what is written in Arabic letters and others.