why is Vne a TAS?

Hello Todd,

In a recent thread on R.A..S.(interesting question about stall and airspeed)
that morphed into a discussion about coffin corner, Todd explained,

"Flutter limits high true airspeed operations, and it is possible to get to
the coffin corner where stall speed and Vne approach each other."

and Jim suggested we "consider Vne as a TAS".

Can I have the groups wisdom and further explanation of the reasons for this
and of how Vne changes with altitude.

Should we be doing preflight calculations of Vne IAS reductions when
planning aerobatics at 9000 ft. or soaring or high speed flight at 15-18,000
ft?

Thanks in advance