There were not spell slots in the older editions, there were just spells.
The line between what's a tweak and what's a more drastic change is in everyone's eye.
Spell slots in 2E were pre-allocated. Say, my Mage knows 10 spells of level 1, has 3 spell slots, and memorises Magic Missile, Magic Missile, Sleep. I have memorised Magic Missile and Sleep. Today, I can cast only 2 Magic Missile, and only 1 Sleep.
Spell slots in 5E are more flexible. Say, my Mage knows 10 spells of level 1, has 3 spell slots, and memorises Magic Missile, Colour Spray, Grease and Sleep. I have these 4 spells memorised. Today, I can cast 3 Magic Missiles, or 2 Sleep + 1 Grease, or any other combination. (It's closer to how Sorcerers cast spells.)
In my view, that's a tweak on the same mechanism.
In BG1-2/2E (mostly), in the Mage Spellbook, the spell slots were on the left, the spells known on the right. I could fill an empty spell slot with a spell known by clicking on the known spell. Or delete the content of a spell slot by clicking on the spell slot. To each their own, but that feels very much like a slot to me, in which I plug-unplug spells.
In BG3/5E, the spell slots are a resource you spend when casting a spell. To each their own, but they feel closer to the standard Magic/Mana Points to me, only more cumbersome to use. (Instead of spells costing variable amounts of MP, your Magic Bar is split in MP blocks that have various sizes and can't be split. A spell can only be cast using 1 block, i.e. spell slot. A level 2 spell slot is a 2MP block, which you cannot use as twice 1MP. Of course, the 2E spell slots can also be called Magic Points, they are just even more restrictive in their use. )
In my view, naming the resource "spell slot" made more sense in 2E/BG1-2 than it does in 5E/BG3.
So, to me, spells slots have been there since the beginning.
