While playing in a buffet world set to end generation I noticed that no phantoms were spawning after not sleeping in a bed for about 4 hours.
I created another world and modified the JSON stats file to artificially increase my time since last rest to over 2 years and still did not get any spawns. I also made sure I was above sea level and had no blocks above me.
I created a platform above the void to prevent other mobs from spawning and impacting the mob cap. Then I turned off the day cycle during the night time so it was always possible for phantoms to spawn.
Still no phantoms spawned.
I used an NBT editor to modify the level.day file so the world generation was set to "chunk_generator":{"type":"minecraft:overworld"} instead of "chunk_generator":{"type":"minecraft:the_end"}. After starting Minecraft again, phantoms started spawning almost instantly and continued to spawn every couple of minutes.
I used the NBT editor to change the world generation back to "chunk_generator":{"type":"minecraft:the_end"} and phantoms stopped spawning. Nothing else was changed or modified. Changing back to "chunk_generator":{"type":"minecraft:overworld"} had phantoms spawning again.
Phantoms should spawn in the over world regardless of the chunk_generator value.
you kind of answered your own bug report here: 6 weeks before the reported version of this report, phantoms no longer spawn in the end
slightly off topic of this bug report:
Main reason I'm bringing anything up is: how do you artificially change how long it's been since you slept. I'm testing to see if Phantom spawning is broken in 1.14 or if I've just been unlucky.