Although recently there's been a rather tectonic shift with regard to the relationship between Windows 10 and "old" versus "new" hardware, even now there's no hard and fast drop dead date with regard to updates, so long as the machine can still get them and meets the minimum requirements. I've got a machine from the Vista era that was running Windows 10, and the only reason I stopped is that, for some odd reason, there was absolutely no compatible mouse pad driver to be found, and believe me I looked at the time. I've got another that's a now at least 10 year old desktop machine that's happily sailing along with Windows 10.
I fully expect that support for Windows 10 will be dropped from machines that run conventional BIOS as opposed to UEFI.
Brian - Windows 10 Home, 64-Bit, Version 1809, Build 17763
A great deal of intelligence can be invested in ignorance when the need for illusion is deep.
~ Saul Bellow, To Jerusalem and Back