Disclaimer, I do not own this particular device, so I can't actually provide the answers to the questions raised.
Before removing this partition, you'd really need to find out what it contains and what it is needed for by the OEM firmware. In general LEDE tries really hard to keep the option of reverting to the OEM firmware without jumping through hoops, so this should be a goal for all changes.
Changing the split between kernel- and rootfs/ rootfs_data partition doesn't affect reverting to OEM, because they are right behind each other, their external boundaries don't change and every vendor firmware update replaces both partitions anyways - this means you can revert to the stock OEM firmware without any loss of data or functionality (you just need to flash the OEM firmware via tftp).
If the contents of the netgear partition were part of these OEM firmware upgrades, so tftp-flashing a OEM firmware would restore the contents of the netgear partition to a sufficient degree, the situation would be the same as before (so removing it for LEDE would be fine) - but that's something you'd have to investigate.
Another option would be if it were used as kind of a cache (e.g. for streamboost, dlna support, etc., so only containing volatile data) and which could be re-initialized by the OEM firmware's initscripts or tftp procedure (this would entail reformatting the partition in a compatible format and restoring the necessary directory structure). In this case removing the netgear partition for LEDE could be considered safe as well, as it wouldn't make reverting back to the OEM firmware any harder.
If neither of the options are available, the situation becomes a lot more difficult - yes, you can say that the user should backup the partition beforehand and keep said backup safe, so they can restore it manually (keep in mind, this would entail a two-step process including an intermediate reversal firmware) if they'd desire to revert to a OEM firmware, but… let's be honest, most users won't notice that requirement early enough - and even if they did, chances are that they'd lose their only backup (also think about second hand devices), which would make reverting impossible. Yes, there are devices where such an approach is needed (e.g. some of the ISP locked lantiq devices), but for those there is no other alternative (the OEM bootloader is locked/ signed and needs to be replaced for anything to happen, at the same time the SOC has a very robust recovery method of booting via serial, without any bootloader involvement), this can't really be said for the r7800 (17 MB for rootfs+overlay are less than expected from a 128 MB NAND flash, but it's still comfortable for many uses).
Disclaimer, I'm not a LEDE/ OpenWrt developer, so the opionions raised are my own and not those of the project.