KJV Exodus 9:19
Send therefore now, and gather thy cattle, and all that thou hast in the field; for upon every man and beast which shall be found in the field, and shall not be brought home, the hail shall come down upon them, and they shall die.
In the above text, we are told that everything that the hail will fall upon will die. The Egyptians who were obstinant left their slaves in the field:
KJV Exodus 9:21, 25
21And he that regarded not the word of the LORD left his servants and his cattle in the field. ... 25 And the hail smote throughout all the land of Egypt all that was in the field, both man and beast; and the hail smote every herb of the field, and brake every tree of the field.
Based on God's promise most of the Egyptian slaves were Hebrews which would entail that it was mostly them who were left in the field.
In other instances during the plagues, the Hebrews were mostly shielded from the adverse effects of these miracles performed by Moses:
KJV Exodus 8:4
And the frogs shall come up both on thee, and upon thy people, and upon all thy servants.
KJV Exodus 8:21, 22
21 Else, if thou wilt not let my people go, behold, I will send swarms of flies upon thee, and upon thy servants, and upon thy people, and into thy houses: and the houses of the Egyptians shall be full of swarms of flies, and also the ground whereon they are. 22 And I will sever in that day the land of Goshen, in which my people dwell, that no swarms of flies shall be there; to the end thou mayest know that I am the LORD in the midst of the earth.
KJV Exodus 9:2-4
2 For if thou refuse to let them go, and wilt hold them still, 3 Behold, the hand of the LORD is upon thy cattle which is in the field, upon the horses, upon the asses, upon the camels, upon the oxen, and upon the sheep: there shall be a very grievous murrain. 4 And the LORD shall sever between the cattle of Israel and the cattle of Egypt: and there shall nothing die of all that is the children's of Israel.
KJV Exodus 10:4, 5, 21-23
4 Else, if thou refuse to let my people go, behold, to morrow will I bring the locusts into thy coast: 5 And they shall cover the face of the earth, that one cannot be able to see the earth: and they shall eat the residue of that which is escaped, which remaineth unto you from the hail, and shall eat every tree which groweth for you out of the field:
21And the LORD said unto Moses, Stretch out thine hand toward heaven, that there may be darkness over the land of Egypt, even darkness which may be felt. 22 And Moses stretched forth his hand toward heaven; and there was a thick darkness in all the land of Egypt three days: 23 They saw not one another, neither rose any from his place for three days: but all the children of Israel had light in their dwellings.
KJV Exodus 11:5
And all the firstborn in the land of Egypt shall die, from the firstborn of Pharaoh that sitteth upon his throne, even unto the firstborn of the maidservant that is behind the mill; and all the firstborn of beasts.
There seems to be no shielding in the issue of hail which would seem like many Hebrews died in this miracle performed by Moses
Were Hebrew slaves killed in the hail?