In the EF you have two prayers, the *Misereator (may almighty God have mercy on us, forgive…) *followed by the Indulgentiam. The SotC is done for the Indulgentiam. Back in the day, people would habitually cross themselves at the end of the *Misereator *in anticipation of the SotC that started the *Indulgentiam *absolution prayer.
After the Council, the powers that be decided to eliminate the Indulgentiam (and therefore its corresponding SotC) from Mass but retain the Misereator. Some argue it was because they were beginning to worry that people thought this prayer absolved form mortal sin. It doesn’t, and to make that clear, they removed it. Whatever reason, this prayer was removed, so a SotC is not called for in the current rite of Mass.
However, you had a bunch of people that were conditioned to make a SotC during the preceding *Misereator *prayer, so even thought the *Indulgentiam *was suppressed, people kept improperly crossing themselves. That habit continues today. Some people think they are being traditional in doing this, when in fact they simply haven’t been informed of the proper gesture at this point in the Mass.