Okay, so I managed to get an old version of swatter working, so here's the issues that it procs up for Grail on the current TBC Prepatch:

Date: 2021-05-28 10:19:26
ID: 1
Error occured in: Global
Count: 1
Message: ..\AddOns\Grail\Grail.lua line 1382:
attempt to call global 'GetFriendshipReputation' (a nil value)
Debug:
[string "=[C]"]: GetFriendshipReputation()
[string "@grail\Grail.lua"]:1382: ?()
[string "@grail\Grail.lua"]:11074: _Tooltip_OnEvent()
[string "@grail\Grail.lua"]:11622:
Grail\Grail.lua:11622
Locals:

AddOns:
Swatter, v1.13.6652 (SwimmingSeadragon)
WowheadLooter, v20501
AccountantClassic, vv2.12.08.20501.00
Armory, vv1.1.1
ATTClassic, v0.5.7
ButtonForge, v1.0.5-classic-TBC
DBMCore, v2.5.4
DBMStatusBarTimers, v
ExtendedMapZoneInfoForClassic, v1.5
FleecingTip, v113.1770(200720)
GatherMate2, v1.46.0-classic
Grail, v113
GrailQuestsretailenUS, v31478
HandyNotes, vv1.6.3
LetMeFly, v1.0
SilverDragon, vv11302.0
SilverDragonClassic, vv11302.0
SlideBar, v1.13.6668 (SwimmingSeadragon)
Spy, v1.1.2
TradeSkillMaster, vv4.10.39
TradeSkillMasterAppHelper, vv4.0.14
Wholly, v079
WIM, v3.9.1
BlizRuntimeLib_enUS v2.5.1.20501
(ck=297)

=================================================================================

And here's the Wholly issue for reference:

Date: 2021-05-28 10:19:26
ID: 2
Error occured in: Global
Count: 1
Message: ..\AddOns\Wholly\Wholly.lua line 3852:
attempt to call method 'SetBackdrop' (a nil value)
Debug:
[string "=[C]"]: SetBackdrop()
[string "@wholly\Wholly.lua"]:3852: _SetupSearchFrame()
[string "@wholly\Wholly.lua"]:686: ?()
[string "@wholly\Wholly.lua"]:2526: _OnEvent()
[string "@wholly\Wholly.lua"]:4427:
Wholly\Wholly.lua:4427
Locals:

AddOns:
Swatter, v1.13.6652 (SwimmingSeadragon)
WowheadLooter, v20501
AccountantClassic, vv2.12.08.20501.00
Armory, vv1.1.1
ATTClassic, v0.5.7
ButtonForge, v1.0.5-classic-TBC
DBMCore, v2.5.4
DBMStatusBarTimers, v
ExtendedMapZoneInfoForClassic, v1.5
FleecingTip, v113.1770(200720)
GatherMate2, v1.46.0-classic
Grail, v113
GrailQuestsretailenUS, v31478
HandyNotes, vv1.6.3
LetMeFly, v1.0
SilverDragon, vv11302.0
SilverDragonClassic, vv11302.0
SlideBar, v1.13.6668 (SwimmingSeadragon)
Spy, v1.1.2
TradeSkillMaster, vv4.10.39
TradeSkillMasterAppHelper, vv4.0.14
Wholly, v079
WIM, v3.9.1
BlizRuntimeLib_enUS v2.5.1.20501
(ck=297)

check out the discussion in #277

Yeah, I'm not sure what's going on in that discussion. Is that a fix, or still being worked on? Coz error lines are different

the changes by @smaitch there should fix it for now.
Can't test it at the moment

I have pushed some more changes today. If you download the latest Wholly and Grail from GitHub I believe they should work. I have tested on the Classic PTR today and it seems to work. Please try downloading them to see if they work for you.

I have downloaded, and installed the latest versions. I only poked my head in, and played around with the settings a little,, but it seems to be working fine. Completion date data isn't available for past quests, but that's prolly more down the Blizzard than the add-on. Nice work ^^

The latest versions of Wholly and Grail are now out officially.