Comment 1 for bug 1805040

Thanks for reporting.

Looks like readline6 was removed intentionally, see https://launchpad.net/ubuntu/+source/readline6/+publishinghistory or https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=877240 for details.

Ideally the applications should be ported to readline7, which is currently supported and available in Ubuntu 18.04. If that isn't possible, you may be able to manually install the older readline6 packages from an older Ubuntu release. Beware that this might conflict with the newer readline version and may cause problems for other packages which now expect readline7.

(I wasn't sure whether this should be converted to a question, so I left it as a bug report)