[One (M8) Android Development] goosnarrggh:
All I'm doing is quoting the official LineageOS wiki maintainers' justification for swapping out all their official documentation to recommend sideloading for everyone on all devices all the time. Nothing more, nothing less.

It is absolutely true that in most circumstances it is possible to avoid sideloading, and experienced users will probably choose to use those alternatives. But the maintainers want a to publicize a single method that works all the time on all A-Only devices.

By the way, the references to Lineage Recovery has nothing to do with recommending using it for the m8. Lineage Recovery for the m8 is a disaster which should be avoided at all costs at the moment. Rather, those references come from the fact that, at the moment, the official LineageOS wiki has been rewritten to support a hybrid approach with respect to how it deals with users' interactions with recovery. The installation instructions for a handful of A-Only devices (but not the m8) have officially switched to explicitly suggesting that users should use Lineage Recovery instead of TWRP, and so they went wholesale through the entire wiki and rewrote everything that talks about working with recovery to make it more agnostic so that it doesn't recommend doing anything that wouldn't be possible with Lineage Recovery.

That introduces some new restrictions. Lineage Recovery can mount SD cards formatted as removable storage, and it can also mount unencrypted data partitions. But not everyone has an SD card; among those that do, some people choose to format their SD card as adopted storage instead; some devices encrypt /data automatically on first boot; and even for those that don't encrypt by default, some people do choose to encrypt /data.

(Personally, I can understand their reluctance to have two sets of instructions, one providing steps to follow if /data is mountable and the other if /data is not mountable. The fewer conditionals you introduce in the documented procedure, the less likely that an inexperienced user might follow the wrong branch and end up stuck.)

Historically, the official documentation has never recommended any techniques that depend on the existence of removable media such as SD cards., and some devices with LineageOS support are incapable of mounting USB OTG drives so those techniques were excluded as well. Now in addition to those restrictions, they also have replaced any techniques which would have depended upon the ability of the recovery program to mount the /data partition.

Their current blue-sky plans, all else being equal, will be to make Lineage Recovery the default recommendation for all devices starting with LineageOS 17.0.

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In my subsequent post, I had moved away from the initial install and on to installing updates. The official LineageOS wiki assumes a working recovery which is able to process downloaded OTAs automatically, and currently something about the combination of LineageOS 16.0+TWRP for the m8 is broken in this respect. The next steps for installing OTAs, therefore, really do depend on whether or not the device is encrypted. I presented two alternatives. The one for unencrypted devices, I think, is the most straightforward: just manually navagate to the directory where the OTA file was automatically downloaded.

The one for encrypted devices deferred back to the official LineageOS wiki, for a solution which is intended to work all the time, for all A-only devices, regardless of the presence or absence of external media, and regardless of which custom recovery you're using.