Can a tech company's software patents be excluded from a sale while everything else transfers?

Yes, in principle, patents are a specific, identifiable asset, and a seller and buyer can agree to carve them out of what's being sold, whether that's a share sale (structured so the patents are held outside the corporation being sold, or transferred out before closing) or an asset sale (where the purchase agreement simply lists which assets, including which intellectual property, are and aren't included). Whether this actually makes sense depends heavily on how central the patented technology is to the business being sold.

If a patent covers the core technology the business's product depends on, excluding it can seriously undermine the value of what the buyer is acquiring, or at minimum needs to be paired with a clear, well-negotiated licence back to the business so it can keep using the patented technology going forward. If the patent is more peripheral, a clean carve-out is more straightforward and less likely to create ongoing friction between seller and buyer.

Getting the scope of exactly what's included and excluded documented precisely in the purchase agreement, and addressing any needed licence-back arrangement explicitly, avoids a dispute later about whether the buyer actually got what it thought it was paying for.