There are topics in the support forum asking about the length of the name of the post types. I remember once I asked myself the same question.
Was it like this since the beginning?
Currently (4.5.2), the schema for the database is located in the wp-admin/includes/schema.php file. At line 162 we can see it that the post_type column is set to varchar(20).
I hunted down when was the post_type introduced since I knew that there were no types of posts. Pages were added later.
When posts types were introduced the schema was located in a different file, namely wp-admin/upgrade-schema.php and surprise, surprise it was varchar(100).
We had 100 character limit until “Some optimization of the post query” was made to the schema, at least this is what the commit message states.
The commit has a closed ticket linked to it, this is where this story ends… with a punch.
It looks like there was a guy with the nickname rboren, who made some suggestion in the #wordpress room and photomatt, aka matt, made sure that is not forgotten.
(11:49:03) photomatt: did you guys see the mysql guy in #wordpress last night?
…
(11:49:53) photomatt: (he also suggested making post_type a shorter varchar, maybe 10-20)
https://core.trac.wordpress.org/attachment/ticket/2604/sql_chat.txt
Later ryan made the modifications – changed the length to 20 – and he made this remark:
Added composite key (post_type, post_status, post_date) and shortened post_type from 100 to 20. Didn’t make a difference to performance.
ticket2604 