I think it was correct that Cancelo was booed when he came on given that he is still a City player, for whom we haven't received a fee and in being brought on could had provided an assist to get Bayern back into the tie at 3-1 (in arguably the most important game of the season thus far). It's also quite clear that his departure mid season was neither planned nor ideal for City. Indeed, with Phillips basically not being trusted the squad size is now very small and we could well be running on empty by the end of May.
In real time at the game, it just seemed not right that he was playing against us and if he had provided such an assist it would have felt worse than "typical City".
If he returns after leaving in the summer for £40m then it would seem churlish not to applaud him for his contribution of the last few years. However for a £60m+ investment to leave unplanned halfway through the season, for no fee, leaving the squad light and being brought on to try and salvage the tie for the German rags, I think it was entirely appropriate that he was booed.