Just to clarify, portamento is the feature that allows synth notes or chords to "slide" from one note to the next. Usually there is a controllable rate that determines how quickly the notes transition. The "Lucky Man" solo is a good example of portamento. This is nothing like a hold or sustain feature, which I think was the question. I have also heard this feature called "latch".

In common additive synth terminology, you have ADSR for attack, decay, sustain and release. If the decay and the sustain were set to the max, then the tones would continue as long as you held down the keys. If release is set to max, the notes would sustain indefinitely even after you released the keys. So an ADSR setting that would be similar to "indefinite hold" is fairly common.

A simpler answer to the Springsteen- Philadelphia question might be that the keyboardist had a keyboard patch with no decay and just held the keys or used the sustain pedal to get the effect. You could also use a sequencer to edit the chords so they sustained as long as you liked and changed when and where you wanted.