This is more of an FYI rather than a useful post, but in preparation for upgrading to 1.14 I've installed a slightly newer version of the backpacks plugin to the server, so it can convert its database to the new style that 1.13+ requires. As far as I can tell, it worked well (as I expected), but if anyone notices any issues with their backpacks, please let me know and I'll restore it from a backup for you.
If you notice no difference, it means everything's worked as it should.
<3