[Baertram]

The text "earned by" is this string constant:
"Earned by: <<1>>", -- SI_ACHIEVEMENT_EARNED_FORMATTER

If you search in the code for SI_ACHIEVEMENT_EARNED_FORMATTER you might find the place where it can be removed:
https://github.com/esoui/esoui/blob/...ments.lua#L861

You can overwrite the string constant with an empty string so the text is not shown anymore:

SafeAddString(SI_ACHIEVEMENT_EARNED_FORMATTER, "", 2)

or re-use ZO_CreateStringId but adding a newer versin 2 should be enough.


Else the tooltip control of achievements are created here:

Could be this control:
https://github.com/esoui/esoui/blob/...ments.xml#L270

ZO_AchievementTooltip

Connected to lua here:
https://github.com/esoui/esoui/blob/...ents.lua#L1480

Object:
ACHIEVEMENTS.tooltip

Function showing it:
https://github.com/esoui/esoui/blob/...ents.lua#L1752
ACHIEVEMENTS:ShowAchievementDetailedTooltip

ZO_PreHook(ACHIEVEMENTS, "ShowAchievementDetailedTooltip", function(selfAchievements, id, anchor)
  return true --suppress call to original function
end)

#Not tested, hope it works and was the correct one

If this was not intended you could also try to prehook the function and change the data in the function accordingly to your needs:

function Achievements:ShowAchievementDetailedTooltip(id, anchor)
end
ZO_PreHook(ACHIEVEMENTS, "ShowAchievementDetailedTooltip", function(selfAchievements, id, anchor)
    selfAchievements.tooltip.parentControl:ClearAnchors()
    anchor:Set(selfAchievements.tooltip.parentControl)
 
    --local progress = GetAchievementProgress(id)
    local progress = 0 --chanegd to always use 0
    local timestamp = GetAchievementTimestamp(id)
    selfAchievements.tooltip:Show(id, progress, timestamp)
  return true --suppress call to original function
end)