It's official, MSE for XP will no longer receive security definition updates. The last one they issued was on April 12, 2016. Manually downloading them will not work either as the update file will not run when you double click it.
BUT, not to worry. If anyone has saved a copy of the definition updates recently (that still worked on XP), they're safe regarding threats dating back before it. As to future threats, lol, the hackers have stopped targeting XP...
Anyways, if anyone knows of a way to extract those definitions from the download package (the newer ones that will only work in Win7) to manually update them ourselves, do post it here...