Wow. Where to even begin naming fighters who have done this... It's almost the rule in boxing, rather than the exception. Recent examples of fighters with reasonable accomplishments who have returned. Floyd Mayweather Jr. (for the McGregor circus act) Beibut Shumenov Robert Guerrero Larry Holmes did it a couple times. I could go on and on and on.
I prefer fighters who announced their retirement and never come back to fight again like Joe Calzaghe and Kotya Tszyu.