What era is it? Why are people still writing traditional diaries?

After traveling to the world of souman, Miyauchi Hikage obtained a magical diary. As long as he wrote in it every day, he could get rewards.
But he had read some diary-based online novels before traveling through time, so he knew that what he wrote would most likely be seen by the heroines of this world.
As a lazy person with no ambition, Miyauchi Hikage immediately decided to be a pure entertainer in this world.
"Who still writes traditional diaries in this era?"
"Is there really anyone who would be willing to write his or her true feelings in a diary and then be watched by countless heroines?"
"My mother told me that this world is very dangerous and boys must protect their privacy when they go out. So don't even think about coming to me. After all, I'm the man you'll never get."
As Miyauchi Hikage continued writing her diary, an "Avengers Alliance" composed of heroines who could see Miyauchi Hikage's diary was quietly born.
They gathered together with only one purpose, which was to find the owner of this diary and brutally torture him!