These days, people seek "religious" guidance from any number of "authorities" and ulemas purportedly knowing more about religion than the average Joe. If a sufficient number of people were to ask about a matter, then the state religious organization may issue a fatwa - or decree - to give guidance on the matter.
Are fatwas spiritual salvation, or are they a pathway to shirk? What does the Qur'an say about this matter?
God's guidance is embedded in all humans
We can see from several verses that guidance is already inspired in all humans
2:97 Say, “Why should anyone bear a grudge against Gabriel? He has only revealed the Qur’an upon your heart by God’s leave
7:172 When your Lord brought forth generations in succession from the Children of Adam, and made them bear witness, “Am I not your Lord?” They said, “Yes, we bear witness.” Thus you cannot say on the Resurrection Day, “We were not aware of this.”
29:49 Nay, here are messages self-evident in the hearts of those endowed with knowledge. And none but the unjust will disregard Our messages.
30:30 Therefore, devote yourself to the Upright Way turning away from all that is false. Such is the nature of God’s creation in all humans. And God’s laws of creation never change. This is the perfect Way but most people do not know.
91:7 Consider the human ‘self’ and Him Who endowed it with the capacity to balance itself.
91:8 And how it is inspired with the capacity to disintegrate or become protected against detriment.
No one is authorized to create religious laws
The Quran does not sanction anyone to create prohibitions or commandments
16:116 Do not keep uttering in falsehood, "This is lawful and this is unlawful”, thus inventing lies against God. Those who fabricate lies against God, fail to prosper in their own ‘self’.
In fact, not even the Prophet was allowed to change any of God's laws
66:1 O Prophet! Would you ban something that God has made lawful for you, even to please your wives? God is Absolver of (their) imperfections, Merciful.
Note that this verse addresses the Prophet and not the Messenger. These roles are significant and extends into narrations about the Prophet, also not being allowed to change the Laws
Other people were also not allowed to create laws, effectively labeling these people as partners of God
3:151 We shall cause dread in the hearts of those who defy the Divine laws since they choose masters besides God. For this, He has revealed no authority, and granted power to none contrary to His laws. Fire is their resting place, and miserable is the ultimate home of those who hurt their own ‘self’ by relegating the truth.
6:81 He said, “Why should I fear what you set up besides Him, when you do not fear to set up besides God, deities for which He has sent no authority and are utterly powerless. Think, which of the two sides among us deserves peace and security? Answer me if you have the answer.”
7:33 Say, "My Lord forbids only: Indecent shameful deeds, open or secret, actions that hurt the ‘self’ and drag down the individual and collective human potential, unjust aggression, associating others with God (Shirk), for which He has sent down no authority, and that you say things about God that you do not know.”
12:40 "Those whom you worship and obey besides Him, are but names that you have fabricated, you and your forefathers. God has revealed no authority for them. All ruling belongs to God alone. And there is no Judge but He. He has commanded that you serve none but Him. That is the Right Religion. But most people are not aware of this fact or disregard it in practice."
53:23 They are but names that you have named, you and your ancestors, for which God has revealed no authority. They follow nothing but conjecture and their fantasies. Now the guidance from their Lord has come to them (hence, they have no excuses)
What are fatwas?
By definition, a fatwa is a "non-binding legal opinion" issued by a "religious authority". From the verses mentioned earlier, it is clear that everyone should decide on matters based on the guidance that is embedded - intellect and conscience - and not allow humans or other agents to usurp God's role in making laws.
From the verses above, if we hold to any man-made commandments or prohibitions as religious laws, then we fall into shirk.