Removing Lightning entirely ... how to? best practices?

It's not documented, but it can be done. This is because Lightning's functionality was split into stand-alone components in version 3.x. If they were using the media functionality only, you can preserve that by bringing in Lightning Media only, even while removing Lightning itself. The main caveat is that in order to do this, the site must have been built with Composer.

Well, the good news is that moving it to Composer is a thing that can be done as well. It's a pain, but it can be done.

So my suggestion to you is:

1) Rebuild the code base on full Lightning, but using the Composer infrastructure.
2) Once that's done, you can restructure composer.json to remove most of Lightning except the stuff you need.
3) Then you'll need to flip a few switches in Drupal itself so that it no longer uses Lightning as the install profile.

At that point, you should be set.

I'm in exactly the same boat, we had a 8.3.x site built by an agency using 2.x lightning. I now need to upgrade to Drupal 8.6.x but am finding the upgrade path troublesome with things like workflow and media.

I also decided I don't want to keep lightning as we don't use it for any of our other sites, and we don't use any features from lightning. Also having lightning seems to require you to stay up to date with the direction lightning is going, and how it all interacts/conflicts with other Drupal modules.

Anyway, so far I've found removing it to be a real nightmare. I was initially going to do this in 2 stages, firstly do an upgrade that removed all the lightning modules, then secondly do a standard Drupal upgrade to bring everything up to date now lightning is gone. With some black magic I've managed to upgrade all my dependencies to the latest versions (I had some pretty tricky dependencies around entity_browser), and remove lightning at the same time.

I've attached to the ticket the upgrade script I've been running to upgrade Drupal then remove lightning modules. There's some hacky stuff I had to do to change the install profile, and clear a value from the menu_tree table that wasn't removed on uninstall. I've also attached the composer.json file we're currently using.

My plan to upgrade is as follows:

- Run this upgrade script on staging.
- Get content editors and site owners to test the functionality of the site.
- Once I'm happy with this, run this migration on staging, and then when it's complete, immediately copy the staging site to production.
- Download the production database and perform a config export as there'll be a huge amount of site configuration that has changed.
- Remove all lightning modules from the codebase.

Hope this helps someone out :)

Here's what I'm considering posting as official documentation for how Lightning can be "uninstalled". Comments welcome!

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Lightning is an installation profile, so there's no "official" way to remove it. The procedure explained here is one that you do at your own risk -- it is wise to do this in such a way that you can test the outcome before you make these changes on production.

1. First, change the current installation profile from Lightning to Standard (or another install profile of your choice), like so: drush config:set core.extension profile standard
2. Uninstall any Lightning modules that you aren't actively using.
3. Export your configuration to account for changes made by the outgoing modules, if needed.
4. You may be using modules that ship with Lightning -- ensure that all of these are explicitly added to your composer.json file, in at least the same version you were already using. For example: composer require drupal/panelizer:^4.2 --no-update, if you were using Panelizer 4.2 or later. This is very important, because your site my break if you remove Lightning before ensuring that all the modules you need are imported by Composer. If you need to continue using a Lightning module, you can import it as you would any other module; for example, composer require --no-update drupal/lightning_media:^3.10. Note that this "à la carte" style only works with Lightning 3 or later.
5. Remove acquia/lightning from your composer.json file: composer remove --no-update acquia/lightning
6. Explicitly require Drupal core in your composer.json file: composer require --no-update drupal/core:~8.7.0. (You should specify the same minor version of core you were using when Lightning was installed.)
7. Run composer update.
8. You should be all set. If you ever reinstall the site, you will need to specify the new installation profile (probably Standard), either in the web installer, drush site:install PROFILENAME, or as a configuration parameter for BLT.

Looks good to me!

Added to README.md in the 8.x-4.x and 8.x-3.x branches, and committed/pushed. Thanks!