If you have extended support: Just update (if it's not so old that it's not even affected in the first place)

If you don't have extended support: You're vulnerable to worse, easier to exploit bugs :)

I have a single 18.04 machine that is stuck on that, because it has 18.04 386, and as 20.04 doesn't support 386 anymore, apparently there is not a simple upgrade path to 20.04 64-bit (without doing extensive surgery). Very annoying...

I can confirm this 18.04 machine still gets some important updates like kernel upgrades and patched versions of Apache.