In my non expert opinion the core series of cpus has not changed dramatically from it's early days. Sandy bridge came out in 2011 and still shares a lot of commonality with current gen comet lake, and suffers from being immensely popular. As such the idiosyncrasies of the architecture have been explored more because they have remained relevant for longer.
The level of research currently being done on these chips would not be carried out on a pentium 4.
EDIT: Just to expand on this, if the popularity of the zen architecture continues i would not be surprised to see a similar 'phenom'enon in AMD's future lineup of cpus occurring. Intel have traditionally produced very solid products and have more resources to identify potential exploits before release. As such i would wager there is some low hanging fruit of this regard in the zen architecture.