I wrote the following on the phpBB.com Discord server (#development):

Hello

We're just wondering about a specific permission, the meaning of which is unclear. So I traced the permission:

Language variable: 
ACL_U_PM_EMAILPM

Permission:
u_pm_emailpm

However, this permission is not used anywhere in the phpBB source. I've tracked all phpBB major minor versions back to 3.0, it's not used anywhere. Either I'm missing something, or this really is an unused permission.

From 3Di:

^^ It is unused. Seems to be dead code.

Reference:

Re: [3.3] Wie "Per E-Mail versenden" unterbinden

Additionally:

This permission was already created in 3.0.0, but not used up to and including 3.3.7.