Can a software company be sold if its code depends on open-source licences with restrictions?

Yes, a software company can still be sold, but open-source dependencies need to be identified and understood before closing, because not all open-source licences are equally compatible with a company keeping its own code proprietary. Some open-source licences are permissive and impose minimal obligations beyond attribution. Others are "copyleft" licences that can require any software built on top of, or combined with, the open-source component to itself be released under similar open terms — which, if it applies to code the company considers its proprietary product, can seriously affect what the buyer thinks it's acquiring.

A software due diligence process for a technology company sale should include a review of every material open-source component in the codebase, the specific licence terms attached to each, and whether the company's use of them is actually compliant. Undisclosed or misunderstood copyleft obligations discovered after closing can be a genuine source of buyer's remorse, since they can undercut the exclusivity of the very product being purchased.

This is specialized enough that it typically involves a technical audit of the codebase's dependencies alongside the usual legal review, not something to leave to a general assumption that "it's proprietary software."