Addons that can be enabled without altering Adventurer gameplay in any way:
(new) Nekarcos's Quality of Life 01: Effect Display
Alternative Combat Sounds
Always center your character
Amiga Music Replacement
Amiga Music Replacement [Pt. 2]
Amiga Music Replacement [Pt. 3]
Auto-Acceptable Acrobatics
Auto-use inscriptions when saturated
Auto-Transmo Gems
Better Item Description
Better Sounds Mod
Crystalist*
Enhanced Object Compare
EquipDoll - Clean Item Names
Go to Landmark
Harbinger*
Improved Auto-explore and Rest
Interracial Masquerade
Inventory Keys
Inventory Sort Order
Juggle Hotkeys on Unwield
Less pinky nightmare curse
Nekarcos's Quality of Life 05: Various (Fatal) Warnings
Nekarcos's Quality of Life 06: Turn Delay
Nekarcos's Quality of Life 07: Visible Size Categories
No Achievement Pop-ups
No move when enemy visible
Notes to Self
Opague Tooltips
Plenum Tooltip: Enhanced actor tooltip display for TOME
Possessor Tweaks
Post Effects fixes
Pragmatic Heroism
Prodigious Progress
Rage Quit
Recognizable Bossess on Minimap
Reflected Golem
Restart Sustains
Ruined Cave Floor Sort
Showing Items Tier in Inventory
Sun Paladin Tweaks
Talent Point Planner
The infamous Wilhelm scream mod
Tinker Tooltips

Everything older than these is older than 1.5 so I'm assuming nobody is using them.

*When not actually playing the Crystalist class or Harbinger class, the only changes made by these addons are that some fixedarts have talent mastery bonuses for Crystalist/Harbinger exclusive trees. These mastery bonuses are useless for other classes and do not affect the cost of the item or anything like that.

I would also argue for allowing Meteor Fix since it's literally just a bugfix and the bugged behaviour usually benefits the player.

Addons that LOOK like they have no gameplay effects but actually do:
Convenient Digging - This was implemented as a sustained talent so it can e.g. tank Entropy (on either players or NPCs).
Improved Player Targeting - Completely breaks various talent speeds.
Mex's Start in Last Hope - Mex is an idiot and this addon breaks Cornac (removing some of their talent points) and probably some other races.