Ooops, sorry for the circular reference. 
The reason for this flag is explained in there: Ghost 5.21 doesn't work with MariaBD - Package frozen at v5.20 · Issue #199 · YunoHost-Apps/ghost_ynh · GitHub
Basically, the Ghost developers have chosen to only support MySQL, while YunoHost uses MariaDB. It has already led to one breakage of the app, which has been patched by yours truly on the package side. But it should be handled upstream by Ghost, and will inevitably lead to catastrophic breakage at some point (IIRC, right now some posts statistcs do not show up, though it is not a critical feature).
Edit: antifeature description updated 