Apart from Bobby Fischer "renouncing", in a sense, the World Championship title in 1975, are there any other famous cases from chess history of strong players voluntarily renouncing titles, tournament victories, and the like, for personal, political, religious, or other reasons?
I have been playing tournaments for 39 years, 34 as a USCF Master, so I follow chess closely; but other than Fischer, I have never heard of anyone doing it voluntarily DURING their reign. Alekhine is the only other male World Champion not to defend his title, but did it involuntarily (he died). :)
Other cases of not defending their title include Hou Yifan, who decided not to defend her Women's World Championship title in 2015 due to another commitment, and after winning it back in 2016, again she chose not to defend in 2017 protesting the knockout system (I do not blame her).
Of course, some have not defended their lesser titles for various reasons, like political and moral. Anna Muzychuk refused to defend her world titles in rapid chess and blitz in 2017 due to Saudi Arabia’s treatment of women.
I have heard of a few, who were stripped of their titles like GM Gaioz Nigalidze for computer cheating (demoted to IM). GM Igors Rausis is most likely facing the same sanctions for his recent cheating incident (He admitted it, and announced his retirement).
A.N Other is not correct. Regardless of the fact that he still considered himself WC, he did, in fact, resign (renounce) his WC title. From the NY Times in 1974:
“I resign my F.I.D.E. [Federation International des Echecs] world championship,” the American grandmaster said in cable received today by the federation, which is meeting concurrently with the chess Olympiad at Nice, on the French Riviera.
The cable was signed “Sincerely yours, Bobby Fischer,” rather than “Bobby Fischer, World Chess Champion,” as he usually writes.
I would have just added a comment, but I am new here, and my rep is not high enough.