The page 163 of the Player's handbook says:
Thanks in advance.
However, even they are an optional rule, feats and multiclasses are allowed in the Adventure League's plays, and some claim they as a core part of the edition. Thus, I want to know how was your experiences without theses optional set of rules. This include gaming balance, martials against spellcasters, fighters without feats and so on.The combination of ability scores, race, class, and background defines your character's capabilities in the game, and the personal details you create set your character apart from every other character. Even within your class and race, you have options to fine-tune what your character can do. But this chapter is for players who-with the DM's permission-want to go a step further.
This chapter defines two optional sets of rules for customizing your character: multiclassing and feats. Multiclassing lets you combine classes together, and feats are special options you can choose instead of increasing your ability scores as you gain levels. Your DM decides whether these options are available in a campaign.
Thanks in advance.