The statues were unheralded for almost a century - a better idiom/phrase

Solution 1:

I think it would be fun, if slightly histrionic, to say that the statues "languished in obscurity".

Solution 2:

Either heretofore unknown or heretofore undiscovered would work if no knowledge of the works existed prior to the discovery.

The works, heretofore unknown, were discovered in the ruins of a monastery in Pisa.

If knowledge of the works did exist, then you could use thought lost or long thought lost, as in

The works, long thought lost, were discovered in the ruins of a monastery in Pisa.

Edit: You can substitute hitherto for heretofore in the first example.

Solution 3:

Since the knowledge of the work did not exist during the period.

What about- "period of anonymity".

The noun anonymity comes from a Greek word meaning "without a name." If you have anonymity, you have namelessness, and people will not know who you are.

an·o·nym·i·ty - (TFD)*

1. The quality or state of being unknown or unacknowledged.
2. One that is unknown or unacknowledged.