Acceptance of contraception, beginning in 1930s, by the great majority of Protestant churches
Acceptance of abortion on demand, by many mainline denominations; euthanasia too
Acceptance of divorce on demand, with remarriage - gradually over decades, by most denominations
Acceptance of same-sex marriage, by some mainline denominations; denial that homosexual actions are objectively immoral
Doctrinally, implicit denial of Trinitarian formula, to some extent, by a few mainlines; for instance, baptism in name of Creator, Savior, and Sanctifier, denial of Father and Son.
The Unitarians, Protestant in 1870, have moved away from Theism.
Some mainlines have moved away from the inerrancy of Scripture, or that is now only an option. The unique role of Christ as Savior is denied, or made only an option.
In terms of the canon of the New Testament, look at the 2013 book The New New Testament, edited by Hal Taussig, with major (name removed by moderator)ut by denominational leaders, including the President of the UCC, Methodist bishops, etc. It changes the language of some of the familiar 27 books, and adds other books among them, as equal “scriptures”. This will be the coming battleground, between churches that hold to the traditional 27 books, and those that put the gospel of Mary on a level with Mathew. Religious leaders took a VOTE to decide which books were scripture for this canon. Other votes will yield different results, adding other books, maybe dropping some of Paul’s epistles.
I think the forerunners of the UCC, and Methodists for instance, would regard the abandonment of the canon as a doctrinal breach.