Excerpt form original text file:

id Software respectfully requests that you do not modify the
levels for the shareware version of DOOM.  We feel that the
distribution of new levels that work with the shareware version
of DOOM will lessen a potential user's incentive to purchase the
registered version.

If you would like to work with modified levels of DOOM, we
encourage you to purchase the registered version of the game. 

So with that said, is it still relevant? It seems pretty unclear or outdated now and would like an official Bethesda (fat chance I know) response to this. Why is it only for shareware levels?

Basically what I am asking is IF we can modify the original levels for our own story telling purposes in a pwad/megawad/wad.

1. Using Doom 1 levels/assets within Doom 2

2. Using Doom 2 levels/assets within Doom 1

3. Final Doom and/or any combination thereoff

4. Making my own version of any of the above.

I know it is a LOOONG shot to getting a response but since I been back into map making as a warm up to SnapMap...