> When will Microsoft stop doing Windows 10 security updates?

Last October, unless you are on an LTS type version - in which case somewhere between 2028-2032 (depending on the exact version). Edge will still update until at least 2028 even though the OS stopped receiving updates... though I'm not sure I would wish either of those usage scenarios on someone :).

> The old laptop does not have UEFI so it could not get the (free for some time) Windows 10 to 11 upgrade.

The free registration, they never actually axed the program at the end date https://techcommunity.microsoft.com/discussions/windows10spa.... Just make sure you use the same edition (e.g. Pro -> Pro). You also don't have to do an in place upgrade to do it. In your specific scenario, you would have to bypass the install requirements in the installer to get around the lack of UEFI though.

Hope that helps, Windows 11 is definitely a bit of an annoying step (even once it's installed).