I think part of the problem is that there's no actual definition of "backwards compatible". 1e and 2e are very much not backwards compatible in the sense that you could mix 2e characters with 1e characters. The 2e characters are far more powerful than their 1e counterparts. The game is balanced significantly differently. Now, if you said, 1e+Unearthed Arcana and 2e, then, well, that's far closer. Although, again, that group is far more powerful than a vanilla 1e group, meaning that the 1e modules are going to be less challenging. At least, that was my experience. 1e modules were cakewalks if you played them with 2e characters at the suggested level.
Or, take Level Up and 5e. Are they compatible? Can I play a 5e Barbarian in a Level Up game? Would it be a large power disparity? I don't think so, but, I'm not all that familiar with Level Up. Those that are more familiar are better equipped to answer that question.
So, in the end, you need to define what you mean by backward compatible before any real conversation can be had.