Despite the various claims, whey protein is halal. It is important to first know what whey is. Whey is” a milk serum, separating as liquid from the curd after coagulation, as in cheese making” [Reference]
Cheese coagulates when rennet is added. So the issue revolves around the permissibilty of eating rennet because whey is the by-product of using rennet. The most correct opinion with the Sahabah radiallahu anahum is that rennet is halal. Even if that rennet comes from the source of dead meat or meat not slaughtered according Islamic manner.
عن عمرو بن شرحبيل قال : ذكرنا الجبن عند عمر فقلنا له : إنه يصنع فيه أنافيح الميتة ، فقال : سموا عليه وكلوه
On the authority of Amr ibn Shurayhil who said: “We mentioned cheese in front of Umar and we said to him: Verily it was made with rennet of dead meat. He said: Mention (Allah’s name) upon it and eat it” [Reported by ibn abi Shaybah in his Musannaf and the narrators are thiqaat (trustworthy].
This is because the Companions radiallahu anhum understood that it has changed in the new product.
Shaikh ul-Islaam Ibn Taymiyyah rahimahullah said:
والأظهر أن جبنهم حلال وأن إنفحة الميتة ولبنها طاهر وذلك لأن الصحابة لما فتحوا بلاد العراق أكلوا جبن المجوس وكان هذا ظاهرا شائعا بينهم
“What is apparent is that their [Zoroastrians] cheese is permissible and the rennet and milk of dead meat is pure. That is because the Companions radiallahu anhum conquered the land of Iraq and ate the cheese of the Zoroastrians and this was apparent and widespread among them”
[Majmoo al-Fatawa (21/103)]
The Permanent Committee for Islamic Research and Verdicts headed by Shaikh Bin Baz were asked:
Is it lawful to eat cheese made with rennet, as such rennet comes from cows and sheep that are not slaughtered in accordance with the Shari`ah (Islamic law)?
There is no harm in eating this kind of cheese. You should not ask about their rennet, as Muslims have been eating cheese made by non-Muslims since the time of the Companions, without asking about such things
You can also the fatwa of Shaikh Al-Albani on the permissibility of rennet here on the blog Darussaafi here.
So if rennet is permissible, then its by-products, like cheese and whey are also permissible. So whey protein is permissible inshallah.
And Allah Knows Best
Faisal Ibn Abdul Qaadir Ibn Hassan