I think we can all be as sure as anyone can be that 10.14 will be the last macOS to support the cMP.
I'm known to be a bit of an optimist, but not unrealistically so. I am not at all sure that 10.14 will be the last version of MacOS for the cMP. In fact I think there is a better-than 50-50 shot that 10.15 will be supported on it too.
And my reasoning for that is that Apple almost never (I haven't actually checked every single release so I'm not confident enough to say never) drops models from its supported list two releases in a row.
Lion supported all the Snow Leopard models
Mountain Lion dropped a bunch
Mavericks supported the same list as Mountain Lion
Yosemite supported the same models as Mavericks
El Capitan supported the same models as Yosemite
Sierra dropped a bunch
High Sierra supported the same models as Sierra
Mojave dropped a bunch
...
Granted, Apple can do whatever it wants but there is usually a specific reason they drop support for an older model (and it's usually related to the GPU). Really, if Apple wanted to drop the cMP 5,1 they would have done it this year (and they still might--as someone else said it ain't official until the GM release) because the stock cards in those machines were all not metal-capable.
I really doubt Apple would suggest everyone with a 5,1 go out and drop $100+ on a new video card and then cut them off from future MacOS upgrades a year later for arbitrary reasons. If the pattern is anything to go by, the next MacOS release probably won't drop any models, and if 10.16 does then it may be for CPU reasons rather than GPU.
Anyway, those are just my thoughts on the matter. People can disagree, but I think the big reprieve for the 5,1 was this year, and next year will just be gravy.