=app-shells/bash-5.0_p17 has a dependency on libreadline.so.8, but puppet-admin is a binary package that has a dependency on libreadline.so.7. The puppet admin package is actually an extraction of the Debian buster package.
The final result of this was I updated the puppet-agent package in my overlay and based my version on the ubuntu 20.04 version of puppet-agent. Seems to work fine, and I could set the RDEPEND to sys-libs/readline:0/8. You can see the result in my overlay at github. The github change really just required changing the deps, and now puppet no longer supports 32-bit packages as of 6.15.
This was a bit annoying to track down, but at least I now know how to use the "--ignore-built-slot-operator-deps=y" option. I found it in the Gentoo Portage FAQ
# emerge -avuDN --ignore-built-slot-operator-deps=y --backtrack=0 @world
These are the packages that would be merged, in order:
Calculating dependencies... done!
[ebuild U ] sys-libs/readline-8.0_p4:0/8::gentoo [7.0_p5-r1:0/7::gentoo] USE="(split-usr) static-libs unicode -utils" ABI_X86="32 (64) (-x32)" 2912 KiB
[ebuild U ] app-shells/bash-5.0_p17::gentoo [4.4_p23-r1::gentoo] USE="net nls (readline) -afs -bashlogger -examples -mem-scramble -plugins" 9948 KiB
[ebuild U ] app-text/texlive-core-2019-r11::gentoo [2019-r10::gentoo] USE="X luajittex -cjk -doc -source -tk -xetex" 12672 KiB
Total: 3 packages (3 upgrades), Size of downloads: 25532 KiB
!!! Multiple package instances within a single package slot have been pulled
!!! into the dependency graph, resulting in a slot conflict:
sys-libs/readline:0
(sys-libs/readline-8.0_p4:0/8::gentoo, ebuild scheduled for merge) USE="(split-usr) static-libs unicode -utils" ABI_X86="32 (64) (-x32)" pulled in by
>=sys-libs/readline-8.0:0= required by (app-shells/bash-5.0_p17:0/0::gentoo, ebuild scheduled for merge) USE="net nls (readline) -afs -bashlogger -examples -mem-scramble -plugins" ABI_X86="(64)"
^^ ^^^^^^
(sys-libs/readline-7.0_p5-r1:0/7::gentoo, installed) USE="(split-usr) static-libs unicode -utils" ABI_X86="32 (64) (-x32)" pulled in by
sys-libs/readline:0/7 required by (app-admin/puppet-agent-6.14.0:0/0::gentoo, installed) USE="-puppetdb (-selinux)" ABI_X86="(64)"
^^^^
It may be possible to solve this problem by using package.mask to
prevent one of those packages from being selected. However, it is also
possible that conflicting dependencies exist such that they are
impossible to satisfy simultaneously. If such a conflict exists in
the dependencies of two different packages, then those packages can
not be installed simultaneously.
For more information, see MASKED PACKAGES section in the emerge man page or refer to the Gentoo Handbook.