Squad uses
TechRequired = Unresearcheable
and so do many mods.

There is an 'e' inside - do not use "TechRequired = Unresearchable".

Also https://wiki.kerbalspaceprogram.com/wiki/CFG_File_Documentation#Editor_Parameters says so.

The only related file I found is:
GameData\InterstellarFuelSwitch\PatchManager\ActiveMMPatches\OnsoleteTanks.cfg

("Onsolete"?)

Please always use
TechRequired = Unresearcheable

^Updated for typos.