With Pascal back in 2016 nVidia introduced the SLI HB Bridge which enabled dual graphics cards only for the GTX 1070, 1080 and 1080 Ti. They actually removed the ability to SLI the GTX 1060 and below probably in order not to cannabalize sales of the higher end GPU. With Maxwell you could still SLI the GTX 960 mid range cards.
nVidia has continued this with Turing and raised the bar as only the RTX 2080 / 2080 Ti have support for NVLInk SLI dual graphics cards. Not even the RTX 2070 has this support making it a very high end premium feature now. This is probably because dual GTX 1070 was a cheap alternative that surpassed a single GTX 1080.