CaveDoctor said:
It’s a misconception that they were lost.
If you look on youtube, there are several rerenders of visual FX from the series that somehow turned up despite being lost.
Some things had to be fixed, but the scenes weren’t lost.
https://www.youtube.com/watch?v=MKj-N5j0SBY&t=1s
The live action footage is upscaled to 4k for this one, but the VFX are all rerendered from original files, with replaced texture maps.

Wow, really cool (and a relief to hear)! Thanks for that info/link 😃