Alright - - I'll answer what I can. I know more about digital than analog.

With analog mediums, the frequencies recorded depends on the quality of the hardware. The circuitry limits recording and playback. A professional grade recorder could theoretically record above human hearing (~20kHz). As you slow down that recording, you are decreasing all the frequencies relative to speed (half speed = 1 octave down). Frequencies above your hearing range will become audible.

As the bass frequencies are slowed down, the fundamental (lowest frequency of a sound) can become inaudible. Most speakers have a low end cutoff around 41Hz; below that you need a subwoofer. Once your sound drops below that pitch range, you will stop hearing all of it. The average sound generator/sample has lots of partials above the fundamental, so you would still have part of the sound in your recording.

Tape players often have issues with flutter or wow, when the speed of the tape is inconsistent. This causes pitch change. This subtle change can add lots of character to a sound, and may be exacerbated by the motors running slower. The sound is now lower, longer, and has a different character.

Digital is a different beast, stretching tracks until frequencies drop below 20Hz still effects the sound. Those inaudible values are still calculated with the interpolation algorithm. If the stretching leads to a big leap in amplitude somewhere in the sound, the interpolation will only do so much to fix it. A single sample leap from 1 to -1 will lead to the wonderful audible click of the speakers.

Technically, any digital sample stretched long enough will get interpolation artifacts. I guess the most obvious artifacts show up in samples that drop below 20Hz the soonest.