That package's functionality was merged into kmod-nf-conntrack upstream, so you simply need to remove that package and it will work fine.
owut upgrade ... --remove kmod-nf-conntrack6
Hmm, might be hard in general. If it doesn't appear in the package build failures list, then you need to dig around.
Here's what I just did on my build machine:
$ git log -n10000 --oneline | grep conntrack6
5b61a50244eb netfilter: remove nf-conntrack6
Aha!
$ git show --name-only 5b61a50244eb
commit 5b61a50244ebc82096f5949de294ad69851e1fd6
Author: Qingfang Deng <dqfext@gmail.com>
Date: 2026-01-19 21:57:46 -0800
netfilter: remove nf-conntrack6
nf_conntrack_ipv6 is no longer a separate kernel module, so remove it.
...
So, maybe go to https://github.com/openwrt/openwrt/commits/main/ and root around?
Or just take the easy way out and post here. It's likely that someone will remember something specific like this one (or at least be able to give you a hint about it).