[Vrul]

Only the last error has anything to do with the changes to this, and only changing this to self won't fix it, you also need to update the function declarations to include that self is passed:

_G.SlashCmdList["GTIPNOTES_SHORTHAND"] = function(input)
    if not UnitExists('target') then
        print(TARGET_ERROR)
        return
    end
    input = input:trim()
    if type(input) ~= 'string' or input == "" then
        if not StaticPopupDialogs["GTipNotes"] then
            StaticPopupDialogs["GTipNotes"] = {
                text = nil,
                button1 = SET,
                button2 = CANCEL,
                whileDead = 1,
                hideOnEscape = 1,
                timeout = 0,
                OnShow = function(self)
                    -- We have to do this onshow to reset the previous text
                    local name = UnitExists("target") and UnitName("target") or ""
                    _G[self:GetName() .. "EditBox"]:SetText(_G.GTipNotesDB[name] or "")
                end,
                OnHide = function(self)
                    _G[self:GetName() .. "EditBox"]:SetText("")
                end,
                EditBoxOnEnterPressed = function(self)
                    addNoteFromPopup(_G[self:GetParent():GetName() .. "EditBox"]:GetText())
                    self:GetParent():Hide()
                end,
                EditBoxOnEscapePressed = function(self)
                    self:GetParent():Hide()
                end,
                OnAccept = function(self)
                    addNoteFromPopup(_G[self:GetParent():GetName() .. "EditBox"]:GetText())
                end,
                hasEditBox = 1,
            }
        end
        StaticPopupDialogs["GTipNotes"].text = POPUP_TEXT:format(UnitName('target'))
        StaticPopup_Show("GTipNotes")
    else
        local name = UnitName('target')
        _G.GTipNotesDB[name] = input
        print(NOTE_SET:format(name, input))
    end
end

The issue with FriendsFrame code is that Blizzard changed it and no longer uses/sets FriendsFrameFriendsScrollFrame.usedButtons:

hooksecurefunc('FriendsList_Update', function()
    local buttons, button = FriendsFrameFriendsScrollFrame.buttons
    local colors, CLASS = CUSTOM_CLASS_COLORS or RAID_CLASS_COLORS, addon.CLASS
    for index = 1, #buttons do
        button = buttons[index]
        if button:IsShown() then
            if button.buttonType == FRIENDS_BUTTON_TYPE_WOW then
                local _, _, class, _, connected = GetFriendInfo(button.id)
                if connected then
                    local color = colors[CLASS[class]]
                    button.name:SetTextColor(color.r, color.g, color.b)
                end
            end
        else
            break
        end
    end
end)

Part of the problem with the first error is that the ammo slot was removed which is what slots[0] is, so remove the start variable and always begin with an id of 1:

local function CreateBorders(frame, prefix)
    local border = { }
    for id = 1, #slots do
        local frame = _G[prefix .. slots[id]]
        local region = GetNormalTexture(frame:GetRegions())
        local texture = frame:CreateTexture(nil, 'OVERLAY')
        texture:SetTexture([[Interface\Buttons\UI-ActionButton-Border]])
        texture:SetBlendMode('ADD')
        texture:SetAlpha(0.8)
        texture:SetPoint('TOPLEFT', region, -1, 3)
        texture:SetPoint('BOTTOMRIGHT', region, 1, 0)
        border[id] = texture
    end
    borders[frame] = border
end

There may still be errors after making those changes, I don't have access to check, but at least you will be one step closer to fixing it.