The new cart reader firmware was added in 4.0.0There's a bit more to it. There are apparently fuses on the game cart reader that were altered with I think firmware 3.0 when Nintendo introduced a new type of cartridge. So you will have to go through some extra steps to preserve the fuses on the cart reader, otherwise you'll lose access to playing games via cartridge if you ever flash back to 2.3. You can find more details on the Switch subforums here at GBATemp.