Background info: I discovered six villains in 2012 and waited for them to get a final fate (that's always a big deal for me). Two were Riptide and Azazel, who disappeared between First Class and the first film, despite being Magneto's first minions, and I was curious as to why.
Days of Future Past reveals that Azazel, Emma, Angel, and Banshee were experimented on and killed by Trask, leaving us to assume the same happened to Riptide. Riddle solved.
Except we never saw Riptide's body, nor did Magneto state he was dead, so they COULD come up with an alternate explanation as to why he disappeared, say he left (lost the will to fight when Azazel died? I ship them) or was imprisoned by someone else introduced in the new film.
Of course, Dark Phoenix is shaky continuity-wise due to age inconsistencies with Charles and Magneto. But will other films fit into canon and say Riptide's alive?
Or will they just forget him on account of being less significant and we still assume Trask killed him with the others?
I'd be inclined to think the latter, but of my other "Mystery Six", two are still alive and likely to stay that way, and the two that died onscreen may be coming back to life permanently, one due to necromancy and the other (no pun intended coz his name is The Other) due to time travel, which recent evidence regarding Guardians 3 suggests will happen even though Spiderman: Homecoming and Civil War will remain intact.
That being said, pretty sure they're gonna retcon Riptide's apparent death to make it a full set (except Azazel, we saw his body, only time travel to before Trask's activities can revive him, and that's unlikely to be done unless they outright retcon Logan or do Deadpool 3). It's all a matter of when. New Mutants? Gambit? What do you think?