My interpretation of out of date interiors is the loud bright psychadelic colors many airlines used in the 70's. That's how I have heard other passengers describe the interiors on some Delta aircraft. Most airlines today use dark blue and gray in their seats, in other words, the colors are more subdued. In the 70s, I know that Eastern, Delta, National, and United used seats of many different colors which were bright and gaudy. Then in the 80s earth tones became the style, where we saw a lot of brown, beige, tan, and rust. The 90s brought in the blues and grays. It seems that most airlines follow the current trend. Some Delta aircraft still have the same bright interiors that were popular in the 70s. I like them, myself.
Fly Eastern's Golden Falcon DC-7B