libssh2 1.8 RPM used to be part of RHEL 8.0. However
libssh2-docs were never included to RHEL and were provided in EPEL. That complicated the usability of this package. So as of RHEL 8.1
libssh2 was moved to the EPEL as well. However it wasn't removed from
modules.yaml metadata file of the repository
rhel-8-for-x86_64-appstream-rpms. That's the problem.
If you want to install the EPEL version, it is necessary to temporarily disable the AppStream repository:
# dnf --enablerepo=epel --disablerepo=rhel-8-for-x86_64-appstream-rpms install libssh2-1.9.0
Then the installation then pass.
The modules metadata available on the RHEL 8.3 installation DVD ISO do not know the
libssh2 and when this media is used as repository, you won't face this problem.
Red Hat is aware of the issue:
The CentOS 8.1+ is probably affected as well, however I haven't checked.