This is what AWS has to say about this:
Oracle has announced the end date of support for Oracle Database version 18.104.22.168 as December 31, 2020, after which Oracle Support will no longer release Critical Patch Updates for this database version. Amazon RDS for Oracle will end support for Oracle Database version 22.214.171.124 Standard Edition 1 (SE1) for License Included (LI) model on October 31, 2020. For the Bring Your Own License (BYOL) model, Amazon RDS for Oracle will end the support for Oracle Database version 126.96.36.199 for all editions on December 31, 2020. All 188.8.131.52 SE1 LI instances will be automatically upgraded to 19c starting on November 1, 2020. Likewise, the 184.108.40.206 BYOL instances will be automatically upgraded to 19c starting on January 1, 2021. We highly recommend you upgrade your existing Amazon RDS for Oracle 220.127.116.11 DB instances and validate your applications before the automatic upgrades begin.
The bit which probably would apply to most of enterprise customers who are running Oracle 11g with BYOL license is this:
|January 1, 2021||Amazon RDS for Oracle starts automatic upgrades of DB instances restored from snapshots to 19c|
Instead of leaving to the last minute, its better to upgrade it sooner. There are lots of things which need to be taken into consideration for this upgrade within and outside of the database. If you need any hand with that, feel free to reach out.