I always get shot down over this but when I took my motorcycle test I used a 1974 copy of the Highway Code which clearly had a blue bike sign denoted "Cycle Lane" (compulsory)
Sadly that was true. I have a paper copy of a Highway Code from 1972 which clearly shows a blue round sign as a compulsory cycle and moped route. I do not know when the compulsion was removed, especially since the round blue sign still indicates compulsion on the continent.
As a matter of interest, you can see both the
1968 and 1974 versions online. The sign of interest is on page 37.
Regarding whether compulsory then and mandatory now are different, of course the Highway Code is not the law. Occasionally, the Highway Code is actually wrong in law, for example
rule 123, "The 30 mph limit usually applies to all traffic on all roads with street lighting...", which has never been true. So, the definition could be an error. What we would be looking for is some change in legislation between 1974 and now. I certainly have no recollection of any such change.