The original Toyota warranty covered the HV battery very well, both for failure and capacity loss (though I'm unsure of what that metric was: the battery proved very durable).
Your extended warranty may cover capacity loss, but AFAIK nobody has tested this and revealed their tale here.
However: generally, extended warranties cover the cost of repairs up to a percentage of the
current market value of the vehicle; if a covered repair would cost more than that percentage (or, sometimes, 100%) of the CMV, the insurer will consider the vehicle a total loss and refund you the remaining balance of your warranty.
Say you paid $3,000 for the extended warranty, and you had already had a typical RAV4 EV failure repaired: a failed cabin heater. The insurer paid Toyota ~$5,000 to have the water heater and DC-DC Converter replaced. If your CMV is $12k, and your insurer will pay for repairs up to 100% of your CMV, then you have $12k-$5k = $7k of warranty "left" to use.
There aren't any Toyota battery repair/replacement options that are going to fit within that $7k window, at least none that an insurer will pay for, so a battery repair or replacement claim under your extended warranty will be met with "vehicle is a total loss" and the insurer would likely give you a payout, possibly as much as $7k, but probably less.
For that reason, if you have a battery capacity issue -- or even a complete battery failure -- once it's out of the original 8-year warranty, the cost of repair is completely in your hands.
If you are interested in some low-cost triage, you might want to
obtain the Tesla diagnostic software (free, for a somewhat-reduced-capability version) and an
adapter cable (at my cost) and let it tell you what it can. It won't show you individual module or cell level degradation, but it can tell you pack capacity, which can be compared with others' here.
If you want more detailed info, ScanMyT3sla (an app for Android & iOS devices, $10), paired with a fairly expensive BT OBDII dongle and a bespoke adapter cable, can show you individual brick voltages, which would verify a sudden capacity loss that could be attributed to a cell/brick/module degradation. More info on this approach is
here.