More learners are available in the
mlr3learners repository on Github (https://github.com/mlr3learners).
There also is a wiki page listing all currently available custom learners (https://github.com/mlr-org/mlr3learners/wiki/Extra-Learners).
A guide on how to create custom learners is covered in the book: https://mlr3book.mlr-org.com.
Feel invited to contribute a missing learner to the mlr3 ecosystem!