Due to performance issues with 1.14, there're certain restrictions to survival that have to be in place to stop the server from becoming overloaded and lagging.
Mojang is working on a performance update for 1.15 so hopefully, in the future, many of these restrictions can be removed.
Mob based restrictions
The vanilla mob cap per player has been reduced for hostile and passive mobs. This means fewer mobs will spawn around a player.
Guardians, Pillagers, Salmon, Cod and Tropical fish are all disabled due to errors in their AI.
Spawner mobs used in mob farms don't have AI. (Don't move, jump, attack, follow players)
Chunk based restrictions
10 Piston per chunk
10 Sticky Pistons per chunk
10 Hoppers per chunk
Each category of a mob is limited to 10 in a radius around each player (Going over this cap when breeding will result in the mob disappearing).
Overworld 12000 x 12000
Nether 5000 x 5000
The End 5000 x 5000
Entity removal is a moderation tool in place that removes all specified entities from the world once every 7 minutes. This stops the overpopulation of many types of common entities. This tool only removes dropped items and aggressive mobs. It will not remove passive mobs or any tamed/named mobs.