Is it true that the reward for reciting the Quran does not reach deceased Muslims, and that the merit of the recitation is diminished if no supplication is made for them after its completion?

This statement is incorrect; in fact, it is an innovation (bid'ah) that must be guarded against. Pure recitation, for the sake of Allah and from a devout heart, is accepted and elevated to Allah without being withheld. This is in accordance with the Almighty's saying: "To Him ascends good speech, and righteous work elevates it" (Quran 35:10), and His saying: "Indeed, Allah accepts only from the righteous" (Quran 5:27). Among righteousness is abandoning innovations and false beliefs. And donating the reward of reciting the Quran to the deceased is an illegitimate act.