Why is fuel adapter removed?

to be fair another one of my many annoyances about the game, is that they removed so many genuinely useful parts.

I know, sometimes the fuel adapter is quite useful, because you can attach nearly everything to it. But you can get it back by downloading an older craft with fuel adapters and then save the fuel adapter as subassembly. That works also with other "old" parts like the crash dummy.

Use fuel line on connecting fuel tanks

It’s obsolete. Engines now have the ability to attach to surfaces on their own.