I think dejavues are glitches in the matrix.
Like even if we live in a simulation, it’s still not 100 percent ironed out and sometimes the code gets corrupted thats why we get those flashbacks from time to time.
We do not live in a simulation, and I have evidence of this.
Brian Greene is a time scientist, he was on Neil DeGrasse Tyson's podcast and he said that time is individualistic.
If life were a simulation, and time is truly individualistic (each observer experiences and actually lives through time differently based on their frame of reference), then simulating such a universe would require astonishing complexity and massive computational resources and storage that is potentially unrealistic, even for an advanced civilization.
Newton said that all clocks are the same for everyone, Einstein said otherwise and was right.
Listen to what Brian Greene says, it's timestamped just press play, he talks about how time is individualistic: