There is already a thread about add-on compatibility, but looking at my extensive spreadsheet of add-on's there are some treasured add-on's that may never be compatible, mostly because the developer has closed.
Some have already been mentioned, but to keep a complete list in one place they are here too.
Dodo-sim Bell 206 - aptly named though Dodo-sim are I think hibernating rather than dead - the most dynamically accurately helicopter in FSX/P3D.
Earth Simulations - ESI definitely gone, the best combined photo-scenery / autogen - far better than ORBX on a small scale but the autogen conflicts with ORBX.
Real Air - also gone, some of the best GA singles and twins in FSX/P3D.
"Realair already works in v4. There were videos showing the legacy running yesterday. The duke v2 uses a 32 bit sound gage which won't work for obvious reasons, but the aircraft itself will function fine"
MilViz say they may not port the F15 or B737 (but not why not)
"older products have outsourced code so they don't own it to recompile it in 64 bit."
Razbam - absconded to DCS, Metroliner would be missed.
Any more to be concerned about?
Cheers
Keith