This is a summary of missable elements in Judge Eyes (Like a Dragon: Judgment). We've also compiled how to recover missed skill books, so check out for missable elements.
Table of Contents
Missable Elements
EX-HARD Difficulty

Only the highest difficulty, EX-HARD, which can be selected after clearing the game, cannot have its difficulty changed during gameplay. Once you start, you cannot change the difficulty until you clear it, so be careful not to overwrite your clear data save.
Skill Acquisition

In Judge Eyes, once a skill is acquired, it cannot be removed from the same save data. SP is valuable in the early game, so prioritize necessary skills and aim to acquire all skills from the latter half of the game to post-clear.
Some Elements Can Be Recovered!
Skill books obtained during the story and time-limited side cases offer another chance in the latter half of the story or after clearing the game. Skill books can be recovered at Onodera's Shop, and side cases in Premium Adventure.
How to Recover Missed Skill Books
Can be Purchased at Onodera's Shop

Skill books missed during the story can be purchased at Onodera's Shop, located in the sewers. At Onodera's Shop, you use SP to make purchases instead of money, so be sure to save up SP through battles and other means.
Unlocked from Chapter 5

Onodera's Shop is unlocked by talking to Onodera in the sewers after the start of Chapter 5. Besides skill books, they also sell valuable items, so it's a good idea to check out their merchandise once it's unlocked.