Is the prohibition in Allah Almighty's saying: (O you who have believed, say not Ra'ina and say Unzurna) general for all Muslims and in all eras, or is it specific to the believers' address to the Messenger, may Allah's prayers and peace be upon him?

The prohibition against saying "Ra'ina" is general, yet it specifically applies to those who misuse the word with the intention of insult. If the reason for the prohibition, which is the possibility of insult, is removed—as is the case in Arabic dialects where it is used during haggling over purchases—then the prohibition does not apply.