SLI — an acronym that originally stood for Scan-Line Interleave, then later for Scalable Link Interface — is, as of today, effectively dead in the form we’ve known it the longest. Modern GPUs that support DX12 support two forms of SLI: implicit and explicit. Implicit SLI is the mode used in DirectX 11, as well as previous versions of Microsoft’s 3D API.
Nvidia made this announcement in the release notes for Version 456.38 of its GeForce Game Ready Software. Beginning on January 1, 2021, no new implicit SLI profiles will be provided for any RTX 2XXX or earlier GPU. All GPU support for SLI going forward will be via explicit SLI, which is to say, the game will have to support the mode directly rather than implementing the mode in-driver.
There’s little reason to expect game developers to consider doing this, however. Of the various Ampere GPUs, only the RTX 3090 has the appropriate bridge connection points. Game developers aren’t likely to invest in optimizing for the tiny percentage of the market that buys such cards, and while they may implement explicit support in a few games going forward to serve existing SLI owners, I expect the feature to die now that Nvidia won’t be providing implicit driver updates any longer.