As stated above, substituting other penances is permitted, not required.
You’re going to get different answers on this one, because it’s a bit complicated. It has to do with Canons 1251-1253 of Western canon law, and the American bishops’ 1966 pastoral letter, which has legal consequences but is not written in a legal style, leaving it open to interpretation.
In the end, I believe we are still strictly bound to do some form of penance every Friday (as well as other prescribed times), because it is the universal law of the Latin Church, even though the USCCB has given U.S. Catholics a dispensation to substitute other forms of penance. But I’m not a canon lawyer, so you may want to ask your pastor for guidance.