CPT symmetry
Charge, Parity, Time symmetry, a fundamental symmetry of the universe. VSL theories may break this symmetry, implying the laws of physics could depend on the direction of time, which is problematic.
Videos Mentioning CPT symmetry

Frank Wilczek: Physics of Quarks, Dark Matter, Complexity, Life & Aliens | Lex Fridman Podcast #187
Lex Fridman
A fundamental symmetry in physics stating that physical laws are invariant under combined charge conjugation (C), parity transformation (P), and time reversal (T).

What If The Speed of Light is NOT CONSTANT?
PBS Space Time
Charge, Parity, Time symmetry, a fundamental symmetry of the universe. VSL theories may break this symmetry, implying the laws of physics could depend on the direction of time, which is problematic.

Why Is CERN Making Antimatter?
Veritasium
A fundamental symmetry principle in physics, combining charge conjugation (C), parity (P), and time reversal (T) symmetry. Violations of this symmetry could have drastic consequences for our understanding of nature.