In the USA, the practice of penance on every Friday (for the Latin Church) is not obligatory anymore, rather recommended (
voluntary). The change dates back to November 18, 1966 norms of the National Conference of Catholic Bishops (NCBB) on penitential observance for the Liturgical Year, which continue in force since they are law and are not contrary to the 1983 code of canon law (see CIC canon 6). That the norms state
voluntary penance on most Fridays of the year was shown in the NCCB document in 1983,
The Challenge of Peace:
298. As a tangible sign of our need and desire to do penance we, for the cause of peace, commit ourselves to fast and abstinence on each Friday of the year. We call upon our people voluntarily to do penance on Friday by eating less food and by abstaining from meat. This return to a traditional practice of penance, once well observed in the U.S. Church, should be accompanied by works of charity and service toward our neighbors. Every Friday should be a day significantly devoted to prayer, penance, and almsgiving for peace.
usccb.org/sdwp/international/TheChallengeofPeace.pdf