The second part of the sixth season is expected to premiere later this year 2020.
Vikings have been slowly losing original characters that have been around since the first season, see Ragnar or more recently Lagertha.
However, the fate of Floki has not been made very clear yet. Many believe that he died in the Icelandic cave where we see him enter and another that will appear in the last round of episodes of the sixth and last season. As many people say: If you don't see a character's dead body in a series, chances are good that it will return.
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Considering that the same creator of the History series, Michael Hirst, has revealed that we will see some of the original characters return at the end of the series, Gustaf Skarsgard has revealed to us the possibility that Floki will appear in the second half of This last installment when asked if he liked the end of his character in Vikings on the occasion of the premiere of his new series Damn:
If you mean if I liked the ending, it was pretty sad. But yeah, I can't say beyond that. But I still see it and it was interesting because I had everything until Floki disappeared into the cave. I read each and every script, but in this new season, I have not read the episodes so it is very exciting for me to see the series without knowing what is going to happen. So I'll keep watching her till the end
The actor thus implies that we may see Floki in some of the last episodes since he cannot "say further" about his character in the series. To see if it will come true, we will have to wait for its premiere, which is expected to do so sometime later this year 2020.