Is some portion of battery capacity reserved?

It pretty much does the same thing as your Prius. In fact, the battery gauge never loses all the bars. There are two bars remaining when it switches to hv mode.

 

I don’t know about the top end, but the manual says it switches to hybrid mode when the charge drops to 12%. I’ve never seen mine go below 8%, so it seems to reserve the same amount of battery capacity as a standard hybrid vehicle (1.5-2kw).

 

The main reason there is a reserve is because of the characteristics of Lithium Ion batteries. If a LI battery is completely discharged it cannot be charged again and is ruined. All hybrid vehicles with LI batteries won't discharge below a point. Some go as low as 10%. A lot of hybrids including two of the 2018 Toyota Camry Hybrids still us Nickel Metal Hydride batteries which can be fully discharged.

 

All plug-in EVs maintain some reserve. The question is just how much. Some reserve only a little; I seem to recall reading claims that some Tesla BEVs may reserve as little as 4%, which I find to be a surprisingly low figure. Some reserve a lot; the Chevy Volt PHEV reportedly reserves as much as 35%!

I think, as a rule of thumb, PEVs with smaller battery packs (less kWh of capacity) reserve a greater percentage of capacity than PEVs with larger packs.

 

I've not seen anything I trust, but you can figure it out from information on the new car sticker. The sticker indicates the car gets 110 MPGe. MPGe is miles/KWh*33.7. If you divide 110 by 33.7 you get 3.264 which is the distance in miles Honda thinks the car will get per KWH. If you divide 47 by 3.264 you get 14.4 which is the usable KWH of the battery. That ends up being 84.7% of the 17 KWH battery.
There is a lot of information at battery university which explains why Lithium Ion batteries are not fully charged or discharged.
http://batteryuniversity.com/learn/article/lithium_based_batteries

 

14.4 kwh is how much from the wall, charging is only around 80-90% efficient so I think usable is around 12 kwh. Meaning the car uses about 70% of the usable capacity, typical of a PHEV to prevent damage to the battery while the generator set/engine is running.

 

Another reason to keep some reserve in the battery is because it is what cranks the engine when you need it--not the 12 V battery.