Are employers required to report the contribution on W-2? If not reported, then what is the effect on the income tax return filed without this information? Do they need to amend their income tax returns?
IRAs are individual (that's what the "I" in IRA stands for). IRA contributions are made by the person individually. IRAs have nothing to do with the employer, and there is no reason an employer would know about or report it.
The short answer is no. Roth contributions are post-tax (not tax deductible). They are often handled directly from the employee's banking setup and the employer has no insight into them. Roth 401(k) and Designated Roth Accounts within a standard 401(k) portfolio seem far rarer than a standard 401(k). (This is just my impression.)
Contributions to Roth plans are not tax deductible. If you have a Roth IRA within your 401(k) (Designated Roth Account), and you make contributions to it through direct deposit, those contributions are not supposed to be included when your employer reports how much you contributed to your tax deductible retirement plan. The contributions are still part of your income. Roth IRA and Designated Roth Account. There's a link on the Roth IRA page that will get you a .pdf showing you the differences between the two.