I did some seaching and came across this well informed thread on the phantom forum. It should give everyone a little peace of mind. Solar Flares
Do keep in mind that the information provided by UAV Forecast in the link kindly provided by patr in the post above may be slightly motivated by their wish for you to subscribe to their (paid) app.
Have to check if KP index is included in NOTAMS, especially since the NOAA site states the KPI can affect GPS, which is used by larger aircraft. Guess if your drone does not use GPS then the KPI is insignificant. Still not totally convinced of it's effect on sUAS, but this is from a guy over half a century in electronics hobby. Good info to know and great posts.
High geo-magnetic activity can degrade reception from GPS Satellites. As long as you are picking up your "normal" numbers of Sats, don't worry about the index. Typically more northern areas are affected, worldwide.
I think Kp index is something to be aware of, but not necessarily restrictive at elevated values. If the values are high pay close attention to your system performance to determine if it acts differently during that period and terminate flights if you suddenly start having problems. We should be doing that with every flight anyway.
This morning shows a Kp index of 3, with a 3 hour forecast of 4.
The effects of a high Kp index will be more pronounced the nearer you are to Earth's Equator and to the poles. That means that, as indicated above, for latitudes going through the likes of the U.S.A. and England, say, the effects won't be so significant and UAS flights in those latitudes would still be possible where a high Kp could be problematic if taking off within the Arctic Circle, say.
For the moderate latitudes, then, one should still be aware and take notice of the Kp index and if it is elevated then take extra care and certainly keep the aircraft in VLOS in case you do lose your satellites.
Through my own choice, since I do tend to be on the cautious side, I generally won't take off with a Kp of more than 4 but I will consider taking off with a Kp=5 if all other conditions are favorable. Definitely no more than 5, though.