Supergirl's Man of Steel, played by Tyler Hoechlin.
Since crossovers are inherent in the Crisis, it's hard to blame fans who are clamoring for the appearance of Smallville's Superman as well. For a generation of fans, Tom Welling's Clark Kent is the definitive Man of Steel, even if he never technically wore the iconic Superman costume. The flagship series of The WB/The CW network for a decade, Smallville has been a major influence on the Arrowverse and on superhero TV series in general. It pioneered recasting DC's comic book icons to be younger and sexier, and it unabashedly provided fan service by guest-starring the late Christopher Reeve and Margot Kidder as well as showcasing its version of the Justice League, Justice Society, and the Legion of Super-Heroes.
However, even if Sam Witwer, who was Smallville's Doomsday, became the Supergirl villain Agent Liberty. While Durance's Lois would likely be the only core Smallville character fans would want to see appear in the Crisis, it would still require some sort of explanation why she looks exactly like the Kryptonian mother of Kara Zor-El (Melissa Benoist) in Earth-38.
Smallville's actors, who are all nearly 20 years older than when the series began, have also moved on. Tom Welling, who just starred in This Is Us.
Unfortunately, Smallville in the Arrowverse would also have to deal with the absence of Chloe Sullivan as his best friend who had a "Wall of Weird". It would be a challenge, to say the least, to explain Chloe's whereabouts in any Smallville revival.
Since 'no tights, no flights' policy and fully embracing DC's superheroes with comics-accurate costumes, superpowers, and tons of fan service. No doubt if Smallville were to become part of Crisis On Infinite Earths, fans would look at it as the long-awaited sequel to Smallville but even an event as big as this crossover would have to focus on the Arrowverse's many main characters, leaving little time to properly explore what the last decade of Superman's life post-Smallville has been like.
While it would be a dream come true for Smallville to be part of the Arrowverse, Crisis On Infinite Earths may not be the right place to integrate that series into the shared universe; it would be too difficult to properly give Smallville its due and would likely leave those fans disappointed and wanting more. Not to mention Smallville's Earth possibly being destroyed in the Crisis isn't something fans want to see.