alphabetized list thanks to TheSteven:

Arturia: use ASC-app “all deactivated on this computer”
BIAB deactivate from within the app
Bitwig: remove host on website
FabFilter deauthorize in app under Help
GForce M-Tron Pro double click in installer
IKM: no de-authorization. You get 10 authorizations and if you run out, IK Support will assist.
iLok: deactivate computer licenses
iZotope: via Preferences in each app
Kilohearts installer app: uninstall all
Korg Legacy: no info
Melodyne: deactivated via website
NI: no deactivation needed
Overloud: deactivated via website
Output Movement: via Movement Uninstall app
OvertoneDSP: hang onto keyfile
Garritan GPO5 & GIO use Aria uninstaller
sonible frei:raum asked for extra activation (since no uninstaller yet)Pianoteq via website
Plugin Alliance: deactivate computer at PA-website
PositiveGrid: uninstallers in downloads
PreSonus: no de-authorization. You have 5 activations. When you run out, request a reset from Support.
My PreSonus - My Products - Choose Product (View More Details) - Manage Activation's (Select appropriate activation and select 'Remove') should never need to request a reset from support.
PSP Audio: no deauthorization but you can use the PSP MultiInstallerPack (uninstaller)
Reason: de-authorized all via website
Spectrasonics: none (“one user/many computers”)
xFer Cthulhu: not even a serial
XLN Audio: unchecked all in Online Installer and removed computer on website
Toontrack: deauthorize via app
Waves: transfer plugins via Waves Central to License Cloud
Zynaptiq (iLok) plus uninstall via app

AudioDamage, d16, Klanghelm, NomadFactory, Novation, Plug&Mix, SugarBytes, SonicCharge, Valhalla, etc.: nothing special