I did 91k miles in a 2015 Golf GTE, over 4 years, and I’d say at least 22k of those were in e-mode. More if you add regen miles etc.
The day I drove it to trade it in, I got 15 electric miles out of it, pretty much the same as I got on day one.
All batteries degrade, we know that much, what doesn’t is the cars usefulness as a plug in electric hybrid, which is what it is.
I would think the Mk8 version, which is what we’re talking about isn’t it, with its bigger battery and buffers will be even less stressed by life as a hybrid.
Even if you put an arbitrary percentage on a battery for warranty replacement, how will you measure it? What will be the arguments when you’re just 1% above that?
Looking at how PHEVs were designed to be used, ie as a fuel multiplier rather than a mini range BEV, then the battery is but one component in a power train system and therefore becomes less important/critical to its through life performance.
As long as you know that the warranty will replace it if fails, then not sure what else there is to worry about.