1. Use this command:

   /give @p minecraft:spawn_egg 1 0 {EntityTag:{id:creeper}}

   You'll get an empty spawn egg.
   If you change "id:creeper" to "id:minecraft:creeper", you get a creeper spawn egg (as expected)

2. Use any of these commands:

   /setblock ~ ~1 ~ minecraft:mob_spawner 0 replace {SpawnData:{id:creeper}}

   /setblock ~ ~1 ~ minecraft:mob_spawner 0 replace {SpawnPotentials:[{Entity:{id:creeper},Weight:1}]}

   Both commands give you a default Pig spawner.
   If you change "id:creeper" to "id:minecraft:creeper", SpawnData and SpawnPotentials are set as expected.

Why would the namespace be required for spawn eggs and spawners? It's inconsistent compared to any other part of the game.
Summon, setblock, selectors, etc. don't require the namespace.

Affects:

• Spawn eggs
• mob spawners