1.8 vs 10

What would people recommend for a new installation?

Should I go with the FreePBX version based on Asterisk 1.8 or the one based on Asterisk 10?

Should I stay away from 10 because support is ending?

Generally speaking, on a new installation it is usually advisable to try to go with the latest long term support stable release, because once everything is up an running, you usually don’t want to touch it with constant upgrades that can introduce hell.
That being said, I (we) went currently go with 11.2 on new installs and have not yet encountered serious issues.

Have some relatively high volumes going on 1.10 and it works just fine. I will be updating it to 1.11 as soon as FPBX v2.11 is ready. If I was adding a new server today I wouldn’t use 1.10 since it’s now a dead end with no more security updates. I would started with 1.8 which is still receiving security updates and then upgrade to 1.11 once FreePBX v2.11 is ready for production. All the Asterisk distributions are mature enough and stable enough and secure enough for production imho so you will be fine no matter what.