> Most security conscious operators would only allow IPv4 instead of IPv6.

I strongly disagree. That's obsolete knowledge, thrown around by people who want to be considered security conscious but haven't actually updated their knowledge in many years. A statement with similar implications might be "security conscious operators require their users to rotate their passwords monthly", which is what a lot of people use to think a decade ago, and now what it really communicates is that you're talking to a dinosaur.

Most IPv4 "security" is a side-effect of NAT. NAT was not intended as a security feature, but it does provide a level of security in the common consumer implementation (PAT, single external IP, unroutable internal network.)

Otherwise, what are the practical differences?

There's more that's been written about configuring/protecting IPv4 networks and if you learn about network security from a mentor or from coworkers at a job, rather than reading books/the internet/LLMs, you're more likely to learn about IPv4 security.

But if you've ever had to deal with endpoint security, if you're just doing static rule based blocking/throttling, /32 or /24 blocking is often your best option. Thanks to CGNAT this often means having a pretty large impact since an ISP's residential customer might end up getting randomly assigned a "bad" address and the cooldown for these rules is often months long or more.

In IPv6 land, you can just block a /128 or even a /64 and rarely hit anyone else in the crossfire.