Most schools continue to follow the NACAC guidelines of a May 1 commit date (Some schools have instituted commitment dates before then and NACAC can not administer consequences to said institutions.)
Regardless the date, a student must accept the offer of admission by the date the college states.
The DOJ imposed rule change that you reference allows schools to continue to recruit students, sometimes by sweetening FA offers, after the student has committed to another school on May 1. I don’t have a sense for how common this practice is, but the new ability to recruit has caused many schools to raise their deposit amount (to deter defection).
Here’s one explanation (there are many on the web):
The First-Year Undergraduate Recruiting Rule prevented colleges, beginning on May 1 of each year, from improving their recruitment offers to first-year students. Specifically, the rule prohibited colleges from knowingly recruiting or offering “enrollment incentives to students who are already enrolled, registered, have declared their intent, or submitted contractual deposits to other institutions.” The rule also prohibited colleges from offering or implying that additional financial aid or other benefits were available to students who had not withdrawn their applications unless the student first affirmed that they were not enrolled elsewhere and were still interested in discussing fall enrollment. The DOJ alleged that the First-Year Undergraduate Recruiting Rule significantly restrained a college’s ability to compete for first-year students, which also deprived students of receiving potentially beneficial offers from colleges.