Before Russian involvement the rebels AA were SA-13, SA-18 and ZU-23-2.
Since Russian involvement they have at least a battery of SA-22 in Donetsk and probably fixed the few non-working SA-11 they had seized from Avdiyvka AD regiment.
The SA-13 is always overlooked despite being a very potent system and very widely used (you've got some in every infantry unit, they're as common as a MANPAD platoon).
F-16 half-load ceiling is 15km.
F-16 loaded ceiling is 11km.
F-16 loaded cruising altitude is 7km.
These are the "hi" portions of a hi-lo-hi interdiction or air support mission. The "lo" or strike portion is below 3km.
This is all from sea level, not ground level, which may be as much as 0.5km higher than sea level on average.
So when it's actually hitting the enemy, an F-16 could be as close as 2.5km to the ground. There's a reason a few were lost to AAA…
Buk can easily reach out and touch you at 20km altitude, and even Kub can reach out to 14km. F-16 is going to fly well within their ranges at all times.
There's no way to escape SAM, you have to kill it.
Another Scorpion killed before her time :.(