I first encountered and used it in the online forums during the late 90’s and early 00s.
Has been part of sentences back then and I still use it like that.
“lol” primarily as standalone reply sounds like something from an later messenger-centric era.
Are you a Millenial by any chance?
gigachad@piefed.social 2 hours ago
Sí, señor. If you like, I can give you my ICQ number.
Multiplexer@discuss.tchncs.de 1 hour ago
LOL.