Both in 1.13 and still in 1.13.1 phantoms do not naturally spawn at night. I can spawn them with a spawn egg and by command.

In both vanilla servers for 1.13 and 1.13.1 i see phantoms spawn at night.

Using craftbukkit 1.13.1 with no plugins, I sit outside uncovered, not sleeping, in survival, non-op, 5+ day/night cycles in a row and not one phantom has ever spawned.

Also tested in 1.13 for a month with 20 players online and a plugin to log any phantoms spawned. Not one phantom spawned and nothing logged unless i spawn one with a spawn egg.

server.properties

---------------------------------------------------------

#Minecraft server properties
#Sun Aug 26 11:52:49 EDT 2018
spawn-protection=0
max-tick-time=60000
generator-settings=
force-gamemode=false
allow-nether=true
enforce-whitelist=false
gamemode=0
broadcast-console-to-ops=true
enable-query=false
player-idle-timeout=0
difficulty=3
spawn-monsters=true
op-permission-level=4
pvp=true
snooper-enabled=false
level-type=LARGEBIOMES
hardcore=false
enable-command-block=false
max-players=50
network-compression-threshold=256
resource-pack-sha1=
max-world-size=50000
server-port=25565
server-ip=
spawn-npcs=true
allow-flight=false
level-name=maps/main
view-distance=10
resource-pack=
spawn-animals=true
white-list=false
generate-structures=true
online-mode=true
max-build-height=256
level-seed=[Redacted]
prevent-proxy-connections=false
use-native-transport=true
enable-rcon=false
motd=Map Since July 2018