as i have learned a few days ago, in the 70s they have used mainly echo chambers, spring reverbs or Plates to create reverb for audio signals and towards the end of the 70s, digital reverbs were introduced.

In the 90s they have sampled stuff from 70s, which was normal for HipHop, so you have only to look at the year of the track release, to maybe narrow down how it was bring to reverberate.

1. How works the echochamber? you play your recording in a room and recording the reverberated sound with a mic. It was 1931 invented in Abbey Roads.

2. Plate reverb you have a metal plate, where you put your signal through and the outgoing signal is the reverb created by the plate (very roughly speaking) - 1957 invented

3. Spring reverb: signal put through a metal spring and outgoing signal is sounding spoky, dark, compared to the plate. (in the 1930s invented)

It was possible to mix Spring reverb with Plate reverb easily to get an even more fuller sound.

here is a good video on this, it is long, but very good explained.

(Its just super mindblowing the first 3 traditional forms on how to create reverb the analog way.)

Conclusion:
The first digital reverb came from EMT in 1976 - EMT250, later the lexicon in 1978 - Lexicon 224. Those are likely to be used towards the end of the 70s, they were still very expensive, but still cheaper compared to the plate or spring reverbs.

The first electronic (with reverb simulation based on microchips) reverb unit came in 1982 out, which could also do non linear reverbs, before they had only plate reverbs, echo chambers and spring reverbs.
The concept of convolution reverbs was even later (couldnt find a year, but Audioease came with the principle in software form around 2001, 1999 by Sony a hardware realtime Convolution reverb - DRE-S777)

Maybe this helps a bit to find more details out.

Sources: https://www.izotope.com/en/learn/a-history-of-reverb-in-music-production.html
Wikipedia ofc