It's not a hard and fast show stopper for me, but it's close. The security updates are why I only have pixels in my family (everyone else's pixel died so they moved to Pixel 3a, but my OG pixel is still going strong). I don't really care about OS updates, so it didn't bother me when my Nexus 5x didn't get Pie because it was still getting security updates for a while when it was still on Oreo. But losing the security updates means I'm done with my OG pixel pretty soon (although honestly, the decreased battery life upon upgrading to 10 had me on that track already). Since they'll release one more update in December, that means the phone isn't "out of date" until the January update that new pixels get. I think I'll keep it around a little while longer until I see if a 4a is going to be released in the spring. If so, I'll get one of those probably.
Honestly, anyone not using a pixel, oneplus, or an iPhone is hopelessly out of date with security updates so it seems silly to be worried about being a few months out of date - but that's where I am.