To answer the question asked -
Automatic enrollment (including EACA and QACA) is treated as a plan feature. Treas. Reg. §1.410(b)-7(c)(1) lists examples of features subject to nondiscrimination testing, and while it doesn’t name auto-enrollment explicitly, features that affect how deferrals are made and how participants participate in the plan clearly fall under this.
The IRS has informally confirmed that automatic enrollment is a BRF. See, for example, the IRS 401(k) Plan Fix-It Guide, which makes it clear that when two plans exist, the availability of different features (like auto-enrollment, loan provisions, hardship withdrawals, etc.) must be tested to ensure nondiscrimination.