Source: Yoda to make appearance in Episode IX

According to “sources” verified as legit by New York Daily News, the upcoming ‘Episode IX’ will be by Rey's side for the final showdown.
“The success of the scenes featuring Yoda in ‘The Last Jedi’ were huge,” says the source. “Yoda will again appear as a ghost as he acknowledges Rey’s success and growth as a Jedi. The scenes are due to be put together later this year.”
Yoda appears in "The Last Jedi" as a Force ghost to give Luke Skywalker a final push for his showdown against Kylo Ren. However, this could be a hint as to Luke Skywalker's future in the franchise, and his being really dead. Yoda seems to now be taking over his duties as a mentor for Rey.