There's everything, from the simplest possible https://sparkjava.com/, to the a little bit nicer https://javalin.io/ (which has a Kotlin wrapper), to the high performance and full framework https://vertx.io/ (which always had great multi-lingual support within the JVM), to the newer and GraalVM-friendly https://micronaut.io/ and https://quarkus.io/ ... for the Enterprise-only crowd, you still have the JEE successor, https://jakarta.ee/ and Oracle's own https://helidon.io/#/ (which likes to position itself as microservice focused, like Micronaut and Quarkus). Not to mention lower level libraries you can use, like https://www.eclipse.org/jetty/ and https://netty.io/.
Ktor has a tough fight to become dominant even in the Kotlin world.
Spring Boot is, still, definitely the most popular option, but it's still just one of very very many!