Seems that the interface tag in the BigDebuffs.toc has regressed. I have installed the addon today (24/09) and this is the content of this file:

---

Interface: 80205

Title: BigDebuffs

Notes: Increases the debuff size of crowd control effects on the Blizzard raid frames

Version: v9.5

Author: Jordon

DefaultState: Enabled

SavedVariables: BigDebuffsDB

OptionalDeps: LibStub, CallbackHandler-1.0, Ace3, LibDualSpec-1.0

X-Curse-Project-ID: 82697

Libs\LibStub\LibStub.lua
Libs\CallbackHandler-1.0\CallbackHandler-1.0.xml
Libs\AceAddon-3.0\AceAddon-3.0.xml
Libs\AceDB-3.0\AceDB-3.0.xml
Libs\AceDBOptions-3.0\AceDBOptions-3.0.xml
Libs\AceEvent-3.0\AceEvent-3.0.xml
Libs\AceHook-3.0\AceHook-3.0.xml
Libs\AceGUI-3.0\AceGUI-3.0.xml
Libs\AceConfig-3.0\AceConfig-3.0.xml
Libs\AceLocale-3.0\AceLocale-3.0.xml
Libs\LibDualSpec-1.0\LibDualSpec-1.0.lua
Libs\LibSharedMedia-3.0\lib.xml
Libs\LibClassicDurations\LibClassicDurations.xml

Locales\Locales.xml
BigDebuffs.xml
Retail.lua
Classic.lua
BigDebuffs.lua
Options.lua

---

Because of this, the game issues an "out of date addon" warning.