Hi,
5.3.6.3 was released in Merchant wave.
The previous plan was to release it in September but we decided to revamp the Roadmap and plans, to provide a more stable version that is required for the future upgrade from 5.3.6.x to 5.4.0.x
Here is the changelog:
https://devs.x-cart.com/changelog/5.3.6.3_-_25_Jul_2019.html
Upgrade from 5.3.6.x to 5.4.0.x will be available no early than the second part of September.
Thanks!