This NIP is not yet in main. Here is an overview of PRs referencing this NIP. 
NIP44 encryption standard, revision 3 OPEN @paulmillr
Introduce NIP-44 encryption standard CLOSED @staab
NIP-44: Encrypted Direct Message (Versioned), replaces NIP-4 CLOSED @paulmillr
NIP-44 Geospatial Types OPEN @rossbates