Can I downgrade to macOS Mojave if I buy the new 2019 MacBook Pro?
Update

Apple has released a new 16" MacBook Pro on 13 November 2019. It replaces the 15" MacBook Pro lineup. The new 16" MacBook Pro comes pre-installed with macOS Catalina 10.15.1 and cannot be downgraded to run macOS Mojave, or earlier releases of macOS.



Yes

So far, year 2019 has seen three releases of MacBook Pro:


13-inch, Four Thunderbolt 3 Ports (May 21, 2019).
15-inch (May 21, 2019).
13-inch, Two Thunderbolt 3 Ports (July 9, 2019).


Source: https://en.wikipedia.org/wiki/MacBook_Pro#Technical_specifications_4

All three of them came pre-installed with macOS Mojave 10.14.5 (However, the build number of macOS for the former two is 18F132, and for the one released in July is 18F2058).

Source: Mactracker macOS app

macOS Mojave saw a point release past 10.14.5 with the current latest public release at 10.14.6.


  Can I downgrade to macOS Mojave if I buy the new MacBook Pro?


So, yes it is possible to downgrade to run macOS Mojave 10.14.5 or later on all the MacBook Pro's released in 2019 till date. Even if the MacBook Pro you purchase comes pre-installed with macOS Catalina 10.15, it would be possible to install and run macOS Mojave 10.14.5 or later as the build includes the necessary drivers for the newer machines.

Note: This above information will most likely not hold true for any newer Mac/MacBook hardware that happen to get released by Apple during the remainder of the year 2019.