Just because it allowed you to install to multiple machines with a retail Win 7 key does not mean it was legal to do so.
Windows OEM licensing has always been one key = tied to the first PC it's installed on.
Windows Retail licensing has always been one key = one PC, transferable to a new PC if removed from the old.
Things like family packs, student versions, VLSC licensing, Dreamspark/Technet, etc all have their own specific exemptions.
Nothing's changed, and what will pass the technical check when you type it in is irrelevant to the legality of it. So to answer the OP's question, Retail Windows 10 is legally good for a single PC with transfer rights.