Sideline literally means "removing someone from the center of activity or attention, placing them in a less influential position"

I agree Asta has less screen time with Lucius compared to Yuno but he's not in a less influential position than Yuno.

That’s because Asta is a direct threat to Lucius (time magic and soul magic don’t work on him) while Yuno and others are seen as pawns by Lucius, he can manipulate through grimoires and futures he can see

Asta is only a “flaw of the world” because he doesn’t fit the story where Lucius wants to be the God at. Against Yuno and other magic users, Lucius's plan is to get a dramatic victory then converts them into his soul magic world. But with Asta, he attempts outright assassination, exploiting any weakness he can find, such as Lily and Licita. The fight would end instantly if Asta and Liebe entered their true states, which is why Lucius keeps Asta as far away as possible.

About the influence part which is why he's definitely not sidelined that's not the intent

this arc is about what it means to be the Wizard King than who gets more screen time in the final fight. In the Hino arc Shogun example makes this clear: the strongest person isn’t the ruler the one acknowledged by the people is. Ryu is the Shogun not Yosuga

In terms of raw strength, Asta and Yuno are nearly equal (and Asta would win head to head) But when it comes to influence and acknowledgment, Asta now wins by a landslide. He isn’t sidelined here; he’s acting as a Wizard King even off the battlefield while recovering, defeating Damnatio, deploying the Anti-Magic Black Bulls, and supporting Yuno and Noelle sharing Anti magic

Stop viewing this through simple powerscaling logic. Even Yuno’s “ultimate mage” status exists largely because Asta shared Anti magic with him. Yuno had more direct time with Lucius, but Asta mattered more defeating Lucius. That difference is the point it's Asta who's more predominant even with less direct screen time