Oracle Instant Client comprises tools, libraries and SDKs for building and connecting applications to Oracle Database. It comes in a number of packages: 'Basic' and 'Basic Light' packages provide a cornerstone, and additional packages can be installed to get extra functionality. Typical uses of Instant Client are for Oracle Database APIs like Python cx_Oracle and Node.js node-oracledb.
As the first yearly release (following on from 18c) in the new Oracle Database numbering system, the 19c development cycle seemed short. However there have been useful changes and fixes. Check out the documentation for the areas you are interested in. One thing I like is the enhanced Easy Connect Plus syntax, which removes the need to have a sqlnet.ora file for some commonly used connection options.
For RPM users, there are two things to note about Instant Client 19c package installation:
The global system library search path is now automatically set during installation of the (requisite) Basic and Basic Light packages. You no longer need to use ldconfig or set LD_LIBRARY_PATH.
You can only have one version of the packages installed by default. For example, the 19c RPMs will conflict with the 18c RPMs.
These two notes do not apply to the ZIP files: you still have to manually configure the library search path to suit the directory you installed into. And you can have as many ZIP versions installed concurrently as you like.
Some old news, but worth repeating is that the RPM packages on yum do not require a login or click through to download.