The Traitors, likely yes. The Tenth Planet 4, no. The Tenth Planet was still being offered for sale in 1974 (AKA the following year) so the negatives still existed at this stage. The 'myth' about the episode going missing snowballed into a legend, especially given the episode in question.
Well, Tenth Planet 4 did go AWOL from the Blue Peter offices. It just wasn't the only copy left (and if it had been sent back to Enterprises at the time they'd have destroyed it later along with their other copies).
Still, it is a pity that Blue Peter didn't mess up slightly differently, by sending The Traitors back to the film library as they should have done, but mistakenly sending 10P4 there as well... Then they'd both be around, as the library didn't destroy any complete episodes after 1969 (they had destroyed at least six before that, three of which have coincidentally been recovered).