Sasuke Uchiha is one of the strongest known characters in Masashi Kishimoto's Naruto series. Born a genius of the Uchiha clan, Sasuke was destined for greatness from a very young age and by adulthood, he managed to fulfill his potential and become a shinobi renowned as a hero for helping turn the tides in the Fourth Great Ninja War. By the end of the war, Sasuke was regarded to be one of the two strongest characters alive, with Naruto Uzumaki being the only one stronger than him.
Even though the main Naruto story ended there, Sasuke's quest to grow stronger certainly did not. Towards the end of Naruto, Sasuke was seen leaving Konohagakure to see the world through his own eyes and redeem himself while he was at it. This journey of his saw him get much stronger than he was during the Fourth Great Ninja War, even though fans never got to see it.
Sasuke During The Blank Period
Sasuke continued his travels across the globe after the end of the Naruto series and, just like Naruto, was actively fighting during this time. While fans didn't get to see much of him during this time, it is a given that Sasuke's control over his Rinnegan increased several times during these few years. It is important to remember that Sasuke received the Rinnegan on the last day of the Fourth Great Ninja War. While he was skilled enough to make use of it, Sasuke's efficiency with it was far from what one would explain expect of a Rinnegan master.
During his fight against Naruto, Sasuke commented on how his control over the eye was improving and that he was finally getting used to it, implying that during his fights against Madara Uchiha and Kaguya Otsutsuki, Sasuke wasn't even at his strongest. With that established, it is quite easy to see that Sasuke did, in fact, power up after the events of the Fourth Great Ninja War in the blank period.
In The Last: Naruto the Movie, Sasuke only makes a very short cameo where he is seen being able to deactivate his Rinnegan at will. In case that isn't an error on the part of the animation staff, it would suggest that Sasuke's control over his eye had gotten much better.
Furthermore, Sasuke was seen casually destroying half of a meteorite with his Chidori in the movie, which just goes to show that his skill was on a completely different level at the time. Surprisingly, Sasuke's major power-ups came after the events of the blank period and closer to the Boruto era.
Sasuke By The Boruto Era
By the time of Boruto: Naruto Next Generations, Sasuke seems to have powered up even more. The very first indication of this is the statement from the author that says Sasuke is equal to Naruto Uzumaki in terms of power by this era. Given that Sasuke lost to Naruto in the Valley of the End and the latter then received another half of Kurama to get even stronger, the gap between the two should've been larger than ever. At the same time, Naruto's control over his powers had also increased to the point where he could one-shot Toneri Otsutsuki and destroy the moon's crust in the process while not even being anywhere close to full-power, clearly indicating a tremendous growth in power.
For Sasuke to be equal to this Naruto Uzumaki in the Boruto era means that he certainly powered up himself and trained tremendously hard to keep up with him over the years. It is further implied that Sasuke's Rinnegan control increased even more by the time of Boruto: Naruto Next Generations. During the Fourth Great Ninja War, Sasuke could only use Amenotejikara using his left eye. In the sequel, he could open portals to different dimensions and travel to every single one of them at his will, showing an increased mastery over the eye.
In the Versus Momoshiki arc, Sasuke was able to fight against Kinshiki Otsutsuki without any trouble whatsoever despite not going all out. Towards the end of the arc, he was able to put up a fight against the fused form of Momoshiki Otsutsuki – a threat that even Kaguya Otsutsuki feared. Not only did Sasuke hold his own against him, but he also managed to hurt him significantly in battle, even going as far as destroying his Rinnegan and trapping him in his own Jutsu. What makes this fight even more impressive is the fact that prior to it beginning, Sasuke carried around 8 people to a different dimension using his space-time Dojutsu. This jutsu is known to take up a significant toll on his chakra when he's traveling alone, meaning taking up to 8 people with him would've had drastically reduced his power.
Despite that, Sasuke was able to go toe-to-toe against Momoshiki Otsutsuki in the fight and once again prove that everything Naruto could do, he could do too. While Sasuke is often nerfed in Boruto, he certainly didn't grow weaker over the years. If anything, the various feats that he displayed across the many fights that he fought over the years clearly indicate that he's grown much stronger.
Sadly, Sasuke lost a chunk of his power when Boruto Uzumaki stabbed his Rinnegan in the Kawaki arc. Now, he must find others ways to make himself more useful in battle, however, knowing the determination and hunger that he has for growing stronger to protect the ones that he loves, Sasuke will, inevitably, stand up once again and grow strong.