Easiest way to instantiate a transceiver in Quartus to avoid unused channel degradation
It seems that adding an IOPLL counts, even though the number of unused channels stays the same:

iopll iopll_i (
  .locked(),
  .refclk(FPGA_CLK2K_P),
  .rst(~A10_RST_L),
  .outclk_0(clk)
);


Routing the reference clock through an IOPLL with multiply/divide settings of 1/1 does not change the clock frequency but allows the transceiver preservation settings to take effect.