It has been nearly 14 years since Left 4 Dead 2 released, and while fans have been crying out for a sequel for well over a decade, this doesn't mean Valve has any intention of making one.
Instead, the company seems focused right now on Counter-Strike 2. Fans are currently pouring through the files of the upcoming shooter, and one user on Reddit found out that there's a Left 4 Dead 3 reference in those files.
The reference reads: "Left4Dead3 version = 2, owner = triage*, severity = high, priority = none, category = ---, product = left3dead3, component = other."
This doesn't seem to indicate much about the game coming about, but a lot of people online think it's quite funny Valve listed the priority as none, implying that there won't be a sequel to the zombie co-op hit for some time.