Retirement Security Act of 2019 This bill modifies various requirements and tax credits for employer-provided retirement plans. The bill modifies the qualification requirements for certain multiple employer plans with pooled providers. The bill applies to defined contribution plans that (1) are maintained by employers that have a common interest other than having adopted the plan, or (2) have a pooled plan provider. Such a plan that meets specified requirements may not be disqualified or otherwise lose its tax-favored status because a participating employer fails to take actions required with respect to the plan. The bill also (1) permits pooled employer plans that meet certain requirements to be treated under the Employee Retirement Income Security Act of 1974 (ERISA) as a single employee pension benefit plan or single pension plan that is a multiple employer plan, and (2) modifies reporting requirements for pooled employer and multiple employer plans. With respect to 401(k) retirement plans, the bill (1) modifies requirements related to default rates for elective deferrals under automatic enrollment plans, the election of safe harbor 401(k) status, and nondiscrimination rules; (2) allows a business-related safe harbor adoption tax credit for small employers, and (3) requires the Department of the Treasury to simplify regulations regarding the timing of participant notices. The bill also (1) increases the limit on the amount of the tax credit for small employer pension plan startup costs, and (2) allows a business-related tax credit for small employers who include and maintain an automatic contribution arrangement in an employer-sponsored retirement plan.