Going further back

Fabrication can exist in recent records too.

I agree.

Age is not the only source of uncertainty.

A recent document may be

forged,

propagandistic,

censored,

incomplete.

A very old inscription may actually be highly reliable.

So historians evaluate evidence by quality, not simply by age.

The general point remains that, on average, surviving evidence tends to become scarcer as one goes further back, but every source must still be critically assessed.