I had to go over the Flirc documentation and other Flirc forum threads. The Flirc documentation made it seem a lot simpler than it was, but various commands were missing when I only followed the Flirc doc, so I had to define commands using 3 different controllers in the Flirc software. As discussed in the doc & other forum threads, it doesn't matter what remote IR commands you use, as long as they are not used for any other devices in your system. I used the old NVIDIA Shield IR codes. I have a Pronto remote, so I was able to copy-and-paste the old NVIDIA Shield codes posted online to my Pronto, and then I used my Pronto to "teach" the commands to the Flirc.
My primary purpose of using Shield TV is for Plex, so I focused on making the commands work for Plex.
Below are the Controllers and commands (in Flirc and the corresponding old NVIDIA Shield IR code) that I used with the Flirc software (again, it doesn't matter what IR codes that you use, but for me, I decided to use the old NVIDIA IR codes):
Flirc Old Nvidia Shield IR Code
Cursor Keys Cursor Keys
Home Shield (other option was Menu). This command seems to take you to home page, as expected.
Flirc Old Nvidia Shield Code
Numeric keys Numeric Keys (0 to 9) <= these actually work for entering in a search window. in the future, consider trying to do alphabet, if it's worth it.
ch+/ch- ch+/ch- <= these behave like cursor up/down. Don't know what scenarios ch+/ch- are useful in. May not be needed?
Shutdown Power On. Although this command is called "Shutdown" in Flirc software, using this command actually turns on the Shield. This command does NOT turn off the Shield.
*Info Display (I used an Oppo IR code for this) <= doesn't seem to do anything.
*Caption Subtitle (I used an Oppor IR code for this) <= doesn't seem to do anything
Flirc Old Nvidia Shield Code
**Rewind Rewind (aside: I believe cursor key left can also be used for Rewind, at least in Plex).
**Forward Forward (aside: I believe cursor key right can also be used for Forward, at least in Plex).
** Note: the Rewind and Forward keys work when paired under Controllers->Media Keys, but don't work when I paired Rewind and Forward under Controllers->Kodi.
Note: Flirc Previous and Next do not work, whether paired under Controllers->Kodi or Controllers->Media Keys.
As indicated above, I got Power On to work. I don't have a way to Power Off the Shield. But not a big deal, since my Shield will sleep automatically after 20 minutes (user-configurable) of non-use. I'd rather have a discrete Power On than a Power toggle, since it makes remote macros more reliable.
The Shield OEM remote also has a settings button on the upper right of the remote. I don't know how to teach Flirc this command, but not a big deal since settings is not used frequently and is easily accessed with cursor keys. However, having this command would be useful if the button is reconfigured to do something else other than settings (such as starting a favorite app).
I also don't have a Flirc command to go with the Netflix command in the OEM remote. But, not a big deal. Kind of strange that Netflix has its own button in the OEM remote. Should have just been made an anything you want command changeable from Settings. Netflix can always be brought up using cursor keys from home page.
Note: before using Flirc, I had plugged in a USB keyboard to the Shield TV to see what keyboard mappings might be useful, but I didn't see any potentially useful commands over and beyond what was mentioned above (if anyone knows any keyboard commands that might be useful, let me know... I recently read that F8 may be useful to get Netflix info while Netflix is playing, but I haven't tried it).
I wish there was some sort of Info command which can be used to bring up Info on the Plex movie that is playing (such as codec, etc.), similar to what can be done when using an embedded Plex Media Player in Linux.
If anyone has other commands not mentioned above, or better ways to configure Flirc, let me know.