How do I know if my PostgreSQL is up do date with DST rules?
Yes, there is a schedule. It's documented here: https://www.postgresql.org/support/versioning/ 


  if need be, security fixes that are released at least once every three months in what we call a "minor release". For more information on the minor release schedule, you can view the minor release roadmap


And the minor release roadmap currently (as of 2019-07-08) shows:


  The target date for these releases are, unless otherwise stated, the second Thursday of February, May, August, and November. The current schedule for upcoming releases is:
  
  &nbsp;&nbsp;&nbsp;* August 8th, 2019
  &nbsp;&nbsp;&nbsp;* November 7th, 2019
  &nbsp;&nbsp;&nbsp;* February 13th, 2020
  &nbsp;&nbsp;&nbsp;* May 14th, 2020


Every minor release will include updated DST rules if necessary. 

The last minor release for Postgres 11 was 11.4 on 2019-06-20. I would have assumed that it included the new rules for Brazilian DST, but I can't find any mentioning of that in the release notes as there was e.g. for the 11.3 and 11.2 releases. 

So most probably, 11.5 (scheduled for August 8th) will include that. 

If you want an authoritative answer from the devs, you should ask that on the Postgres mailing list



Postgres can be compiled in a way that it uses the operating system's time zone data. You can check if your Postgres version was compiled with the --with-system-tzdata option by running pg_config