Was cache automatically disabled activating maintenance mode?
Short answer: No, it shouldn't have.
Enabling maintenance mode shouldn't do anything to caches. It is only a mechanism to stop the bootstrapping of Magento during deployments or when you want to prevent users from getting to the website.
Magento Dev Docs - Enable or disable maintenance mode
What happens when activating maintenance mode?
When you activate maintenance mode, Magento will prevent the application from launching. There is a maintenance mode check in vendor/magento/framework/App/Bootstrap.php that calls $this->assertMaintenance(). It will throw an exception if maintenance mode is enabled, halting the running of the application.
This does what you would expect; Prevents users from browsing or shopping the website.
Pros/Cons to activating maintenance mode
Pros
- Prevents requests to the website from being fulfilled during Deployments / Deployment Rollbacks
- If you were performing a heavy upgrade/migration/feature deployment, keeping the site in maintenance mode would give you time to perform any manual steps before users start hitting the website
- Prevent new orders/transactions if a serious production site issue is happening and you need to firefight / troubleshoot
Cons
- Stops orders/transactions from coming in
- Stops users from being able to browse/interact with the website
- If activated long enough, effecting SEO crawls since all page requests will result in the maintenance mode page
- Users bouncing from the website since they are receiving maintenance mode page
Known Issues With Cache Automatically Getting Disabled
I'm not sure exact version numbers affected, but it is a known issue with certain versions of Magento that the caches get disabled during setup:upgrade and for whatever reason don't get re-enabled (I'll try to find the GitHub issues associated). For a few clients I've worked on this would happen during deployments and I'd have to make a note to explicitly log back into the server and enable caches after the successful deployment.
Links to check out related to cache disabling problem
Magento Github Issue - cache disabled after run "composer update" command
Magento StackOverflow - Magento 2.2.x cache disabled automatically