Having looked it up, I think what’s happening here with the word knackered is 2 separate words converging:
First and Original meaning and totally acceptable usage:
Knacker = old Norse for saddle
Knackers yard = slaughterhouse
Knackered = slang for exhausted / worn out.
Second use, according to the source I found citing the Oxford English Dictionary that this first started to be used in the 1970s (I had never heard of this latter use and I was around in the 70s). NB the first use above appears in the OED.
Knackers = slang for castinets
Knackers = changed over time to slang for testicles
Knackered = shagged out
As for crap, this is not a swear word.
First and original meaning = rubbish ie waste product but not excrement.
More recent meaning (the original meaning being altered whilst also keeping the original meaning) = excrement ie waste product. Possibly popularised because of Thomas Crapper American, 19th century maker of toilets, but first used mid 18th century when he was a child.
Crap is therefore a perfectly acceptable way to say rubbish in slang without reference to poo. Ditto knackered without sexual connotation.