Yeah, that acronym sorta keeps nagging at me too, but unfortunately I haven't been able to make any real progress.. It is complicated a bit also by the fact that Espressif refers to it as both "SLC" and "SCL" in different headers (I'm assuming one is a typo, but it's been cut/pasted so much it's hard to tell which is the correct term).
A friend of mine more familiar with such things did mention that "SLC" is apparently also the name of an on-die processor-interconnect technology (though I can't seem to find any references to it online), but it's unclear whether that has anything to do with this or not (one theory I had, though, was that the ESP8266 chip internally uses an SLC interconnect bus to connect the CPU to a DMA controller, which is why it got the "SLC" name some places, but that could be completely off base).
I suppose if we're really curious, somebody could try asking Espressif what it means.. you never know, they might just give us an answer.