LMK04610: Is driving the SYNC input with a sine wave ok?

I have some reservations about using a sine wave instead of a square wave at the SYNC pin:

• If the slew rate is too low, the LMK04610 could experience high current consumption during the threshold transition intervals. I'd recommend at least 0.1V/ns to avoid any prolonged shoot-through current on the pin input buffer. Note that this shoot-through current isn't going to damage the device; it just increases the current consumption proportional to the amount of time spent in the shoot-through transition.
• If the slew rate is too low, the SYNC signal (which is re-timed by the VCO post-divider, which generally has pulses at 1-2GHz i.e. 1ns to 500ps) may not have deterministic alignment with the VCO post-divider retimer. For instance, at 0.1V/ns peak slew rate, for a 1.8V sine wave, this corresponds to around 17ns for a 20% to 80% low to high transition and thus 17 to 34 post-divider cycles. The threshold will be a narrow band within that range, so in practice the uncertainty is closer to two or three post-divider cycles, but there is still uncertainty, particularly part-to-part where the threshold voltage may vary due to process or temperature. This may or may not matter for your use case.
• If the frequency at the SYNC pin is too fast, the desired SYNC behavior may not be complete before the next SYNC event is triggered; in other words, the setup or hold time may be violated. I did not find anything in the datasheet documenting the setup or hold time requirements for the SYNC pin, so it's not clear how fast this signal could be retriggered or if there are even setup and hold times to violate; but if the sine wave signal is continuous or more than one pulse, the SYNC event will likely be triggered multiple times at best, or the signal chain through the SYNC may be left in an unplanned state at worst. My best guess for the setup and hold time is at least three VCO post-divider cycles, based on counting flip-flops in a simplified internal design diagram - in practice, this number may be slightly larger or smaller.

All that said: if you are unconcerned with, or unaffected by, the reservations outlined above, there is no particular electrical risk to the device from driving the SYNC pin with a 0V to 1.8V sine wave.