A Tangerine Dream Live Question

That was definitely true. I was taught electronic music starting in 1975 on a modular Moog, and if you wanted to 'remember' a sound, you'd have blank 'patch cord charts' that would have all the plugs jacks and dials on the Moog, and you'd mark the settings on the dials with pen, and draw in which patch cords were going from which jack to where. Even then it was rarely precise. I'm less familiar with old sequencers, as I never learned on those.

By the late 70's and early 80's you had synths with presets, but they were often a small number of cheesy factory sounds that I doubt people used.

I still have a Jupiter 4 , which was one of the first polyphonic synths. It has a mind of its own. Sounds different every time you play it. Also takes it at least 10-15 minutes to get in tune after you turn it on, and that's assuming there haven't been any temperature variations.

So early on, TD were probably almost obliged to improvise to a degree whether they wanted to or not. The 77 North American tour is the first one where they actually started playing anything like set pieces. Shows from that tour have the four tracks from 'Encore' in the first half of the show, and then the second half is  improvised, differing wildly from gig to gig. I think Froese famously put on virtually a metal guitar improv when they were in Dertroit.

By the early 80's they were playing more consistently structured/composed pieces. I'm not clear when this became more or less completely set. I thought it was pretty early in the 80's but some other posts here suggest not.

1984 was when the digital change hit, and the Yamaha DX7 came out, with a big onboard memory of digital presets. But they were an absolute cow to improvise on. It was FM synthesis, which is really complicated and threw everything I'd learned out the window - but most of all the interface was just a bunch of touchpads. No knobs, no sliders - this was pretty much how it went all through the rest of the 80's and a lot of the 90's. Of course TD were using a lot of more exotic equipment than that.

They've gone back to their roots now, ironically post-Froese - and doing completely improvised stuff as part of their shows again. The three live  'Sessions' CDs they've released (that I know of) since about 2017 are worth checking out.