Also, Season 5 has shown villains can still emerge even when timelines change, so perhaps The Red Death was a pre-crisis enemy that may surface post-crisis due to timeline altering.
With that said, the Arrow-verse is known to take liberties with character origins, so it’s possible Barry won’t be the speedster The Red Death merges with to gain the powers of the Speed Force.
The character’s canon dictates he’d need Barry in order to gain control of the speedster’s body.