You must be thinking of forced IDs, the feature itself is several years old but it's been turned on and off through the years.
Something that isn't very well known is that 4chan is actually a lot of smaller communities with only partial overlap, and the culture can differ a lot depending on what board you are.
A strangely effective rule of thumb is that the smaller the community is, the friendlier it acts.
IDs are only enabled in the bigger and most controversial boards of the site, where people otherwise don't hesitate to abuse their anonymity.
The cost of registering – even if it's just 3-4 clicks away – is not negligible, and it gives a very different situation by default. I won't discuss the merits of a reputation & voting system, but when everyone has an account, the actual result is that few people are anonymous in practice. And maybe that's not so bad.
But on imageboards like 4chan, you will find very few people using names or even so-called "tripcodes" (the closest thing to an account), which means that by default to anyone that isn't a moderator, everyone is Anonymous in a sea of Anonymity.
Of course every Internet forum keeps logs of what IP posted what, but as long as it's kept private, that's entirely besides the point.
Something that isn't very well known is that 4chan is actually a lot of smaller communities with only partial overlap, and the culture can differ a lot depending on what board you are.
A strangely effective rule of thumb is that the smaller the community is, the friendlier it acts. IDs are only enabled in the bigger and most controversial boards of the site, where people otherwise don't hesitate to abuse their anonymity.