Tonight at Night Two of Castle Attack 2021 Kota Ibushi who is the current IWGP Heavyweight and Intercontinental Champion defeated Tetsuya Natio in the main event. With this win in only his third defense of the Intercontinental Title he officially unified both belts.
Here is a quote from Tokyo Sports:
The history of IWGP will change! At the IWGP Intercontinental (IC) Championship (28th, Osaka-jo Hall) of New Japan Pro-Wrestling, champion Kota Ibushi (38) succeeded in defending for the third time, rejecting the challenge of Tetsuya Naito (38). Following the results of the IWGP Heavyweight and IC double-crown unification controversy, Chairman Naoki Sugabayashi (56) announced in this paper that he would unify the two belts as the two-crown champion Ibushi hoped. A new belt that inherits the history of both thrones, the name "IWGP World Heavyweight Championship" will be newly established.
After the match El Desperado who just won the IWGP Jr. Heavyweight Title challenged Ibushi to a match at Nippon Budokan Tournament on March 4th.
The IWGP Intercontinental Championship was established on January 5th, 2011 and there have been 15 champions since then with the inaugural champion being current WWE Superstar MVP.
This now leaves the IWGP United States Championship currently held by AEW star Jon Moxley as the only midcard title in NJPW.
RIP IWGP Intercontinental Championship (January 5th, 2011- February 28th, 2021)