It is not a fee per se but it is a restriction of the transaction fees.

1sat/byte wasn't the minimum fee in the past, free transactions were possible. As long as your transaction cannot meet the criteria of a free transaction, there must be a minimum fee for the transactions to be relayed by the nodes with default parameters. Since it's defined in the default settings, changing it by the owner would also result in some transactions being accepted (if decreased) or some transactions being rejected (if increased). This is specific to individual nodes but the 1sat/vbyte is assumed to be adopted by most of the nodes on the network and thus it is the minimum fee for a good propagation.