If a specific sentence is not in-text referenced but the website used is mentioned on the reference page, is it considered plagiarism?
Yes, it is. When you omit the citation, you are claiming the text as your own. Any time you use the words or ideas of another you need an in-text citation which points to a reference that has enough detail to allow others to find the source. If you are using the exact words, you also need quotation marks.