I assume you're talking about The Roku Channel and not Roku as a streaming platform.
Seasons 5-8 are currently on The Roku Channel.
Keep in mind that, like on every streaming service, movies and TV shows or TV seasons will rotate off for a variety of reasons, so a show could disappear from The Roku Channel as easily as one could disappear from Netflix. It's a matter of streaming rights, which are for a limited period of time, and library concerns (how many can reasonably be offered without become a mess of confusion).
DBDukes Roku Community Streaming Expert Note: I am not a Roku employee.
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