I have specifically not added that out of respect for what Slirp represented to dialup shell users in the late 1990s and for what it did for the ecosystem in the past 15 years. I used it for more than a decade myself, and if you consider that it's now used for containers and virtual machines, given the original purpose, it's really an impressive piece of software. Feature comparisons can be risky at times.
It's mostly in the direction of IPv6 support, performance, security. Those topics are covered in slides: https://static.sched.com/hosted_files/kvmforum2022/01/passt_... or recording: https://www.youtube.com/watch?v=U89bWP1HNgU of a recent talk from KVM Forum 2022.