While I think many folks here would argue that assembler is outdated in general, the specific form of assembler used in the Voyager probe is probably considered outdated by any reasonable standard.
Assembly is not outdated in general, it's just outdated as a human interface for big CISC programs. If you are working on very small micro-controllers it's not even necessarily outdated as a human interface since you might be working on a much more minimal platform in which case it's important you understand exactly what's going on.
What was probably meant in the article is that the particular assembly used in the CPUs of the CCS at the time is quite different compared to various modern micro-controller assembly (not compared to CISC like x86 or even ARM which are almost completely different beasts).
Sometimes these projects have custom silicon and so are even quite different to other commercial chips of the same era... Having a quick google, the CCS was NASAs first redundant computer, having two of everything, but at a fairly low level unlike say the mars rover with it's triple redundancy basically at the "whole computer" level. Interesting stuff, I wish there was more info available about all the details of these machines.
The program was written more than 40 years ago, most of the people who wrote it are probably retired now.
Also, it's not a "mainstream but old CPU" like a Z80 or a 6502, given the constrains of a space probe it's probably more a custom thing developped specifically for these (kinds of) missions.
If you were a young engineer at the time, let say 22, you are now 65 years old, if you were 30, you are now 73.
So there is a good chance that people who worked on the design and creation of these probes are now retired, except maybe for the youngers (25 and less or the time).
It's absolutly not a statement against young versus old. By 68 years old, I hope to be retired ^^.
And it doesn't take into account that the computer system is derived from the Vikings probes, so it's even a bit older than that.
As for the assembly language, I don't think they are stating that assembly is outdated by itself. I think that they are more stating that the assembly language used for these probes, custom and purpose made for these probes, coming fom the 70ies, is now outdated.
And assembly can still be a very legitimate choice, specially in highly constrained domains and high safety assurance requirements (even trusting a compiler is a risk).
https://www.wired.com/2013/09/vintage-voyager-probes/
Now, the compiler back end for the CPU may not exist any more, but I wouldn't think that would be all that hard to reproduce.