First, Celie chooses to remove Mr. ____'s last name from the text of her own private letters. If she "must" refer to her husband without his last name, it is due to her own psychological and emotional needs. This is not a rule put in place by Mr. _____. That should be clearly understood because removing the last name is part of Celie's power, part of her character, and later part of the demonstration of her growth. 

Removing her husband Albert's last name from the text does mimic the style used in older books wherein the name is eliminated for the purpose of creating a sense of realism. There is also a political and character-driven explanation. 

Celie is a powerless woman at the novel's outset. Her avenues for self-expression and empowerment are limited. They are effectively limited to her letters. In writing her letters, Celie is free to speak openly. This is a kind of power. Given this power, she erases the identity of her husband, obscuring or obfuscating him much as he does to her in daily life. 

Mr. ______ say How old she is?

He say, She near twenty. And another thing-She tell lies.

By not using his name, she is exercising her power. 

Ultimately, she does begin to use Mr. ____'s first name, signifying a change in her character. Celie's growth in confidence means that she can now deal directly with her problems without resorting to evasion or obfuscation. 

She has gained power in her real life (by giving it to herself, learning confidence and self-love).