The answer is no.
Joseph Smith Jr. was succeeded by others who also received "thus saith the Lord" oracles. His successors Brigham Young and John Taylor were also the recipients of "thus saith the Lord" oracles.
The Prophet isn't the only person who can receive such oracles either. However, any person claiming to receive such for anything other than his own direct stewardship responsibility should bring them to the Prophet in order to have what they claim as revelation to be ratified as such and thereby become binding upon others. In actuality, Joseph Smith Jr. was instrumental in establishing a system of authority that makes all worthy men into prophets over their own stewardships.
It is also important to understand there is more than one level of ministration that is possible when a Prophet/prophet is receiving revelation. Some of the oracles given to Joseph Smith Jr. and his successors were spoken by the Father and some were spoken by the Son. In LDS theology this is understood as Celestial and Terrestrial ministration respectively. According to D&C 76 when you keep the laws of the Celestial Kingdom you are entitled to receive ministration from the Father and when you keep the laws of the Terrestrial Kingdom you are entitled to receive ministration from the Son. It is also understood that all mankind is currently able to be ministered to by the Holy Ghost, which is understood as Telestial ministration.
Unfortunately, the Church has undergone some difficulties and is currently awaiting its redemption such that for the time being it only qualifies to receive Telestial ministration. This is why there hasn't been a "thus saith the Lord" oracle in the church from the Father or the Son for well over 100 years. Church leaders retain the authority of their offices but they are only able to get Telestial level ministration in the performance of their duties. This will not change or improve until the laws of the Terrestrial and Celestial Kingdoms are fully reinstituted and practiced by the Saints.