Is Apple Watch 5 compatible with iPhone 6+?

Since Apple Watch 5 will come with WatchOS 6, you would need at least an iPhone 6s to use it.

See this:

watchOS 6 compatibility.

watchOS 6 requires iPhone 6s or later with iOS 13 or later, and one of the following Apple Watch models:

• Apple Watch Series 1
• Apple Watch Series 2
• Apple Watch Series 3
• Apple Watch Series 4
• Apple Watch Series 5

Not all features are available on all devices.**

ONLY if you never update to WatchOS 6. This would not be a wise move. iPhone 6 was released in 2015 and is now a completely unsupported iPhone. It is capable of running iOS 12.4.1, but it will get no more iOS updates, like iOS 13 coming out next Thursday. You will really be limiting yourself if you stay with your iPhone 6. It would make more sense to move up to a more modern iPhone, THEN add Apple Watch.