Hard to answer correctly as there are many grades of wire. But yes I have used flex to make links. Of course should be right colour or sleeved and correct thickness.
The only problem is with maintenance free connections. i.e. where there is a spring rather than a screw.
As to why one needs a link is another question?
Years ago it was common to use twin instead of triple cable to link between switches. As a result the live was borrowed from an adjoining switch. However this has two major problems.
1) Because the feed and return are not in the same cable it can produce mains hum on radios and TV's in the house. Even the phones can be affected.
2) It is easy for the live to be taken from a different circuit (Circuit is all supplied by one fuse or MCB) to that feeding the other switch. This results in a borrowed neutral situation which is dangerous.
So rather than asking if a bit of old flex can be used the question is why do you need a link?
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