Russia tried hard to appease Israel before Il-20 incident
We all know about cruise missile strikes on Syrian army bases, airfields and installations because of supposed use of chemical weapons. But Israeli strikes were going on almost routinely throughout the war and did much more damage to Syrian army and their allies, but didn't get much coverage in the media .
Russia always tried to placate Israel (reasons for that are numerous and warrant question of their own) including cancelling delivery of S-300, turning blind eye to Israeli attacks (it is now clear they had some sort of communication with them that they didn't share with Syrians) , giving respect to Israel by inviting Israeli PM to Victory Day parade etc ...
Things would probably continue this way, but loss of valuable Il-20 EW aircraft with trained specialist crew is huge psychological blow. It is not the first incident where Russia lost personnel by direct action of Western powers and did not retaliate (loss of Su-24, attack on "Wagner" soldiers, few other suspicious incidents with loss of aircraft ...) It appears that not only they cannot protect their Syrian allies, they cannot even retaliate when their own troops die.
Russia fears that if they just let this pass, next attack on their military would be even more devastating. So now they started to draw the line . Syria formally gets S-300. Although Syria did send some Air Defense specialists to be trained on this system some time ago, it is reasonable to assume that Russian advisors would help them, at least in first few months of operational service. So the stakes are getting higher. S-300 is not state of the art SAM any more, but it is still very dangerous against any 4th generation fighter planes, and those planes (F-16 and F-15) make a bulk of Israeli fleet. What is even more dangerous is that Russian S-400 already deployed in Syria (plus maybe even their fighters) could give support to formally Syrian air defense systems. Russian already promised to jam communications and GPS signal with their Krasukha-4 and other EW assets.
Ball is now in Israeli court. We shall see is this enough to deter them, or do they actively push for conflict vs Russia.