hook_boot() and hook_exit() are no longer called on cached pages. Per the new doc-blocks for these functions, the previous functionality can be re-enabled in settings.php, but is discouraged. For all contributed module purposes, there is now no way to fire events on cached pages.
This is an intentional change to increase performance and consistency with all other caching systems (e.g. a module like Boost, a reverse-proxy like Varnish or Nginx, or CDNs like Akamai and Cloudflare).

Introduced in branch: 
1.0.x
Introduced in version: 
1.0.0
Impacts: 
Module developers
Related Github Issues: 
https://github.com/backdrop/backdrop-issues/issues/493