This is a good question. Some people may read one ayah and from it just make conclusion that is not correct. Shirk is indeed forgivable by Allaah swt but this depends on repentance from the person from shirk.
Allaah says in Surah Al maidah concerning the christians who have worshipped eesaa a.s and said that eesaa a.s is actually God(Allaah). This is a lie and shirk.
They have certainly disbelieved who say, " Allah is the Messiah, the son of Mary" while the Messiah has said, "O Children of Israel, worship Allah , my Lord and your Lord." Indeed, he who associates others with Allah - Allah has forbidden him Paradise, and his refuge is the Fire. And there are not for the wrongdoers any helpers.
ayah72 surah al maidah
and the next ayah is also about the christians who say God is one of three. They have come up with a big lie and shirk claiming Allaah is one of the three instead of saying God is ONE.
They have certainly disbelieved who say, " Allah is the third of three." And there is no god except one God. And if they do not desist from what they are saying, there will surely afflict the disbelievers among them a painful punishment.
after Allaah exposed their errors, lies and shirk/kufr then in the following ayah Allaah gives them hope of forgiveness even from shirk and kufr. And Allaah encourages them to repent to Him alone from all the lies, shirk and kufr.
So will they not repent to Allah and seek His forgiveness? And Allah is Forgiving and Merciful.
This is indeed enough evidence that Allah is not only forgiving of shirk provided the one who committed repented and sought forgiveness but also encourages the committer of shirk to seek the forgiveness of Allah alone because He swt is the most forgiving and most merciful.
but to add another two ayahs even though the ayahs are plenty to prove Allaah forgives even shirk but the condition for the committer of shirk is that he/she MUST repent and ask for forgiveness from Allaah before death. If someone dies in a state of shirk without having repented from the shirk, then He/she will NOT get forgiveness from shirk that is because forgiveness from shirk is conditional upon repentance and seeking forgiveness from shirk before death comes(before the soul reaches the throat in death) or before the end of the world when the sun rises from the place it sets. As for other lesser sins such as stealing etc then Allaah will if He will forgive even without repentance before death for whomever He wills.
The other ayah is in surah AlFurqan towards the end.
ayah 68 surah 25
And those who do not invoke with Allah another deity or kill the soul which Allah has forbidden [to be killed], except by right, and do not commit unlawful sexual intercourse. And whoever should do that will meet a penalty.
Multiplied for him is the punishment on the Day of Resurrection, and he will abide therein humiliated
Except for those who repent, believe and do righteous work. For them Allah will replace their evil deeds with good. And ever is Allah Forgiving and Merciful.
so in essence even if people invoke another deity(shirk) then they forgiveness and repent a sincere repentance from even shirk then Allaah will forgive and exchange them their sins with good deeds. Look at the mercy of Allaah its truly amazing.
Lastly from surah Az zumar
ayah 53 surah 39
Say, "O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah . Indeed, Allah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful."
This ayah clears up any misunderstanding because Allaah forgives ALL sins even shirk BUT, because this includes shirk and even kufr then condition upon forgiveness of shirk is still there as Allaah goes on to say in the following ayahs.
And return [in repentance] to your Lord and submit to Him before the punishment comes upon you; then you will not be helped.
and the ayahs after this ayah continue to stress on the need for repentance. So, with the Quran and Sunnah you have to compile all the information and then make a conclusion otherwise you will have a misunderstanding. Wallaahu a'lam.