While Momoshiki Otsutsuki had a huge part to play in the Boruto: Naruto Next Generations manga, the same hasn't been the case for the sequel series, Boruto: Two Blue Vortex.
The antagonist last appeared in the final chapter of Boruto: Naruto Next Generations. Since then, he has only been mentioned a few times in the sequel series with no real plot development around him.
While many fans have theorized why Momoshiki Otsutsuki might be absent from the new series, a new theory has speculated that the Otsutsuki might be waiting for the right moment, similar to his fellow Otsutsuki clan members.
Disclaimer: This article may contain spoilers from the Boruto and Jujutsu Kaisen manga and reflects the author's opinion.
Momoshiki's absence in Boruto: Two Blue Vortex may have something to do with his plan

As per the latest Boruto theory from YouTuber @Kashy_Kage, Momoshiki's absence from the plot in the Boruto: Two Blue Vortex manga may have something to do with the trait that is seemingly ingrained in the genes of every Otsutsuki.
It is the same trait that helped Isshiki Otsutsuki revive himself nearly a thousand years after he arrived on Earth, i.e., patience.

As fans must have learned from the manga, Isshiki Otsutsuki had arrived on Earth 1000 years before the events of Boruto alongside Kaguya Otsutsuki to harvest a Chakra Fruit. As Kaguya was a lower-ranked Otsutsuki, she was to be sacrificed to harvest the Chakra fruit. However, Kaguya betrayed Isshiki and left him on the verge of death after destroying his lower half.
Despite being on the verge of death and no longer capable of implanting Karma, Isshiki Otsutsuki persistently turned himself small and entered the ear of a monk named Jigen. Years later, Jigen established the Kara alongside Amado and worked on his mission to revive Isshiki Otsutsuki by implanting Karma onto a new vessel, which turned out to be the series deuteragonist Kawaki.

Similarly, after his previous attempt to take over the protagonist's body got botched, Momoshiki might also be patiently waiting for the right moment to play the most unexpected card.
It could be possible that Momoshiki may have again foreseen a future where his host was guaranteed to lose his mind. Thus, this time, he may be staying quiet to leave the protagonist unprepared for a moment, either as horrific as the destruction of the Hidden Leaf Village or maybe the death of all his loved ones.

As evident from the series, the Otsutsukis are a very patient species and are willing to wait for centuries to fulfill their plans. Thus, it would not be wrong to assume that Momoshiki was also waiting for the right moment, similar to how Ryomen Sukuna from Jujutsu Kaisen waited for the right moment to switch bodies from Yuji Itadori to Megumi Fushiguro.
When Megumi believed that he had lost everything, Sukuna hijacked his body by forcing him to eat his finger. Similarly, Momoshiki might hijack Boruto's body when a similar moment arrives.
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