And this error:

Message: Interface\AddOns\GuildGreet\GuildGreet.lua:1314: attempt to call global 'GuildRoster' (a nil value)
Time: Wed Oct 14 07:30:03 2020
Count: 9420
Stack: Interface\AddOns\GuildGreet\GuildGreet.lua:1314: attempt to call global 'GuildRoster' (a nil value)
[string "@interface\AddOns\GuildGreet\GuildGreet.lua"]:1314: in function `GLDG_OnUpdate'
[string ":OnUpdate"]:1: in function <[string ":OnUpdate"]:1>

Locals: self = GuildGreet {
0 =
}
elapsed = 0.017000
setting = true
(*temporary) = nil
(*temporary) = "attempt to call global 'GuildRoster' (a nil value)"

This likely has to do with them changing a lot of the api calls anymore.

GuildRoster(); has been changed to C_GuildInfo.GuildRoster()

Okay, it looks like (and please correct me if I'm wrong, ShowFriends() has been changed to C_FriendList.ShowFriends().

Which seems close to fixing things... but now the violent neon green background has disappeared altogether, with no background whatsoever (making it really hard to read the text)

And next, it seems that canViewOfficerNote() was changed to C_GuildInfo.CanViewOfficerNote()

Which seems to have removed the LUA errors, but I continue to be irked by the lack of a background, although the addon is starting to work like normal otherwise.

Okay, next change seems to be CanEditOfficerNote() needs to be changed to C_GuildInfo.CanViewOfficerNote()

By the way, officially thank you to lownignitus - you gave me just enough of a hint to start tracking these down, and I've been changing them on my local version. Successfully up to now, it seems. But looks like SetBackdrop() is no longer a used call, (with no replacement I could find) which I THINK is the reason the background is gone altogether

It was already fixed in the last patch at 9.0.1.
But gOOvER packed the wrong scripts together.

Why did you open the same post twice? Look here:
#43

Everything was already discussed in your first post.

Okay, having attempted to apply the changes from GuildRoster(); to C_GuildInfo.GuildRoster() - the following seems to be the next problem.

Message: Interface\AddOns\GuildGreet\GuildGreet.lua:1245: attempt to call global 'ShowFriends' (a nil value)
Time: Wed Oct 21 11:28:40 2020
Count: 1
Stack: Interface\AddOns\GuildGreet\GuildGreet.lua:1245: attempt to call global 'ShowFriends' (a nil value)
[string "@interface\AddOns\GuildGreet\GuildGreet.lua"]:1245: in function GLDG_InitRoster' [string "@Interface\AddOns\GuildGreet\GuildGreet.lua"]:238: in function GLDG_OnEvent'
[string ":OnEvent"]:1: in function <[string ":OnEvent"]:1>

Locals: name = "GuildGreetFrameSettingsGreeting"
(*temporary) = nil
(*temporary) = "attempt to call global 'ShowFriends' (a nil value)"
L =

{
Remove selection = "Remove selection"
List to chat when a player levels up (offline players, printed when you log in) = "List to chat when a player levels up (offline players, printed when you log in)"
achievment = "achievment"
Use local time instead of server time = "Use local time instead of server time"
Enable player context menu (may cause taint issues in raid frame, see tooltip) = "Enable player context menu (may cause taint issues in raid frame, see tooltip)"
SubTabDebug = "Debug"
Set guild collection = "Set guild collection"
has increased his level from %s to %s = "has increased his level from %s to %s"
No characters found = "No characters found"
Say see you later to guild and channel = "Say see you later to guild and channel"
Supress all = "Supress all"
Don't put players coming online on the greet list = "Don't put players coming online on the greet list"
At %s, %s promoted this player to rank %s. = "At %s, %s promoted this player to rank %s."
Custom collections = "Custom collections"
List grows upwards instead of downwards = "List grows upwards instead of downwards"
Printing information to chat = "Printing information to chat"
Say goodbye to guild and channel = "Say goodbye to guild and channel"
Selected collection = "Selected collection"
ChatMsg/The config string seems to be corrupted. Please generating a new one. = "The config string seems to be corrupted. Please generating a new one."
Display your own characters = "Display your own characters"
List alt and main names when player logs in = "List alt and main names when player logs in"
Current value = "Current value"
ChatMsg/Config string not found. = "Config string not found."
Using chat frame %d (%s) = "Using chat frame %d (%s)"
Write the config string = "Write the config string"
Only show level-up for levels above %d = "Only show level-up for levels above %d"
At %s, this character came online for the first time during this session. = "At %s, this character came online for the first time during this session."
Channel name to monitor = "Channel name to monitor"
Test trigger = "Test trigger"
(off) = " (off)"
Whisper greetings and grats to players = "Whisper greetings and grats to players"
Don't put players that get promoted on the greet list = "Don't put players that get promoted on the greet list"
coming online = "coming online"
Add main name to chat when an alt sends a message = "Add main name to chat when an alt sends a message"
Show /who request and response text in chat = "Show /who request and response text in chat"
later channel = "later channel"
List alt and main names when player logs off = "List alt and main names when player logs off"
GuildGreet = "GuildGreet"
Configuration options for displaying the players waiting for a greeting = "Configuration options for displaying the players waiting for a greeting"
Character reached level %s. = "Character reached level %s."
bye char = "bye char"
Clear greet list = "Clear greet list"
Greet guild when 'Greet Key' is pressed (also applies to saying bye) = "Greet guild when 'Greet Key' is pressed (also applies to saying bye)"
Create new collection = "Create new collection"
Collection %q = "Collection %q"
Only update guild roster on events = "Only update guild roster on events"
Whisper level up messages = "Whisper level up messages"
At %s,