Important Note [MAY 2020], the updated dates can be found here:
As a DBA you need to regularly check and review Oracle Support Matrix especially if you have a big infrastructure with mixed Oracle database releases. Also, its very important from a cost perspective as (it depends on your contract with Oracle), to avoid “Paid Extended Support”.
What can be read from different Oracle resource 18c & 19c are considered under 12.2 release umbrella.
To the date of writing this blog (December 15 2018), the following are support dates:
So 19c is considered the long term support !
Release Schedule of Current Database Releases (Doc ID 742060.1)