There were sorcerers in BG2 and they were a core class in 3E. I do not know anything about 4E, but it's wrong to say sorcerers are new.
I do not know DnD 1E and 4E, but there are spell slots in 2,3 and 5E.
Sorcerers were implemented in 3E which was after Mystra died. They are in BG2 which happens right before Mystra dies the second time, but that is a deviation from the rules. There was not a sorcerer class in 2E (at least not in the original 2E rule set)
There were not spell slots in the older editions, there were just spells. Casters got a certain number of spells of a given level but could only cast spells of that level and spells scaled on caster level not on spell slot. For example a 3rd level wizard got 2 first and 1 second level spell in 2e/3e. They could cast 3 magic missile spells (assuming they were memorized) and would get an extra missile in each by being a 3rd level wizard. However they could not use their 2nd level spell to cast magic missile. They could not upcast using a slot, upcasting was based on caster level. A 20th-level wizard using magic missile would fire 10 missiles but would still cast it as a 1st level spell.
5e allows you to cast any spell prepared (or known) using a "slot" of equal or higher level. That mechanic did not exist in 1E, 2E or 3E. My 3rd level 5e wizard has 4 1st and 2 2nd level slots. He can use all 6 of them to cast magic missile. When he uses a 2nd level slot he upcasts it getting an extra missile, but when he uses a 1st level slot he does the same damage and has the same number of missiles as a 1st level wizard. Same with a 20th level wizard even. The 20th level wizard has 4 first level spell slots and his magic missile using one of these is the same as a first level wizard's magic missile.
