The increasing mess of
libreadline versions in Linux distributions
GNU Readline is normally supplied and used by programs as a shared library (even if it's possible to statically link it, almost no one does). Some or perhaps many of those programs are not from the distribution; instead they're your local programs or third party programs. Shared libraries have major and minor versions (and also symbol versioning, but let's ignore that for now). The minor version of a shared library can be changed without upsetting programs linked to it, but the major version can't be; different major versions of a shared library are considered to be entirely different things. If your system has libreadline.so.6 and libreadline.so.8, and you're trying to run a program that was linked against libreadline.so.7, you're out of luck.
(Major shared library differences are required by ABI differences even if the API is the same and the program's source code can be immediately rebuilt against a different version of the shared library with no code changes.)
Unfortunately, two things are true. First, the GNU Readline people apparently do things that change the ABI on a regular basis, which causes new versions of the shared library to have new .so major versions (again, on a regular basis). Second, Linux distributions are increasingly providing an incomplete set of past libreadline shared library versions. This came up with Ubuntu 18.04 and libreadline.so.6, and recently I discovered that Fedora 30 has moved from libreadline.so.7 to libreadline.so.8 and not provided a compatibility package for version 7 (although they do versions for readline 6 and readline 5).
(I'm assuming here that the shared library version is changing due to genuine ABI incompatibility, instead of just the GNU Readline people deciding to call their latest release 'readline 8' and everyone following along in the .so versions.)
Just as with Ubuntu, the net effect is that it's impossible to build a local binary that uses GNU Readline that works on both Fedora 29 and Fedora 30, or even that can survive your system being upgraded from Fedora 29 to Fedora 30. If you upgrade your system, you get to immediately rebuild all programs using GNU Readline. I don't think you can even install the Fedora 29 readline 7 RPM on a Fedora 30 system without blowing things up.
It's my strong opinion that this overall situation is asinine. Linux distributions are not KYSTY environments, where the system packages are only for system use; a Linux distribution can reasonably expect people to build software against system shared libraries. Right now, using GNU Readline in such software is pointing a gun at your own foot, and not using GNU Readline is annoying for people who use your software (people like those readline features, and for good reason).
(At the same time, the Linux distributions are not the only people you can blame. The GNU Readline people are presumably unlikely to do bug fixes and security updates for GNU Readline 7, because they've moved on to GNU Readline 8. Linux distributions don't want to have to take on the burden of maintaining a long tail of GNU Readline versions that are no longer supported upstream.)
As a side note, it's very easy to miss that this has happened to some of your binaries if you only run them once in a while. I generally assume that Linux binaries are quite stable and so don't run around testing and rebuilding things after Fedora upgrades; generally I don't even think about the possibility of things like missing shared libraries.
PS: In their current development versions, Debian appears to have both libreadline7 and libreadline8; older versions of GNU Readline seem to be more spotty in general. The current stable Debian has libreadline7.