Almost all software that Debian packages is 3rd-party. The issue is usually that software like Sublime Text or VS Code is non-free. That is not in itself an impediment for being packaged; after all there is the "non-free" section of the archive. However, often non-free software is also not free to be distributed by third parties. Thus, Debian would break the law if they did.
You don't need to adapt your software that much to have it run on Linux distributions; there are standards that the distributions implement that you can rely on. Often software that claims to only support one particular distribution will run perfectly fine on others. Linux distributions are not unfriendly towards third party software, but they have no obligation at all to spend effort to make that software work, it's the third parties that should do that work.
The bug tracker being email based is because when Debian started, that was the normal way to communicate on the Internet (besides IRC). A lot of tools were built on it, and the Debian developers themselves are used to it, so there is little incentive to change this.
The Debian developers would say that apt is not buggy; it's just that if there are conflicts, they have to be resolved in some way, which means deleting some of the conflicting packages. It also does ask you to confirm in this case. Although it would indeed be better if it would detect this is a very unsatisfying solution.
You don't need to adapt your software that much to have it run on Linux distributions; there are standards that the distributions implement that you can rely on. Often software that claims to only support one particular distribution will run perfectly fine on others. Linux distributions are not unfriendly towards third party software, but they have no obligation at all to spend effort to make that software work, it's the third parties that should do that work.
The bug tracker being email based is because when Debian started, that was the normal way to communicate on the Internet (besides IRC). A lot of tools were built on it, and the Debian developers themselves are used to it, so there is little incentive to change this.
The Debian developers would say that apt is not buggy; it's just that if there are conflicts, they have to be resolved in some way, which means deleting some of the conflicting packages. It also does ask you to confirm in this case. Although it would indeed be better if it would detect this is a very unsatisfying solution.