Imagine a Game Jam event but you can only use software which is 10 years old or older.
For example:
We could go deeper and limit software which is 20 years old or older. So no Git!
Gonna bring back memories and make us appreciate goodies we have in 2019! 
Kinda cool, although it may be a little hard to find that software around (also I donāt think anyone will install a different OS to participate in a Jam).
But what are you expecting? Most of the software you quoted is (in the context of use in game jam), more or less the same, just loads faster. The only one that is significantly different now is Blender.
I am in no goddamn way nostalgic for Unity 3 or Blender 2.49. Let the past stay buried, where it belongs.
I think the biggest challenge would be coming up with workarounds for missing features (e.g. Mecanim, which didnāt come out until version 4).
I think such restriction may result in good old 2010-style games appearing (worse graphics with hacky mechanics etc.)
Not sure about others, but you could get Unity 3.4.0 from the download archive, but I remember downloading version 1 some time ago!
And yeah, downgrading OS would be hardcore.
Oh, come on! Donāt say you didnāt enjoy pressing āBuildā by accident and not being able to cancel the build progress? And donāt get me started on $1,500 shadows!
Is the theme of this game jam going to be āmasochismā??? 
Agreed. Ten years isnāt enough. We need to go back thirty. How about C64 Studio? 
http://www.georg-rottensteiner.de/en/index.html
Edit: Yes, I know the app isnāt thirty years but thereās no way any of us will want to use just the machine.
Could use a vm.
Ha! Thatās a nice workaround! But, depending on machine, it may be slower to work in VM.
True.
Yāall think too small.
Or, more specifically, too big.
Sourcesafe and VB6. Those were the days. NOT. fucking no. Never going back