I like the sound of the phrase and I wanted to make a shirt design with it. But as soon as I started googling for some existing ideas, I found that google and duckduckgo just refuse to search for "Til death do us apart" without quotations and change it to "Til death do us part". So I wonder if its grammatically correct to say "Apart" in this context.
Proves that it is correct are: movie "Til death do us apart" (2018) and various songs titled with "Apart".
Proves that "Part" is correct: the actual oath, wiki and various songs titled with "Part".
The problem with "Part" is it does not sounds as "cool" as "Apart" for me (personal preference), but at the same time I wouldn't do a shirt with incorrect spelling either.
UPD: The rewording of the phrase to "Until Death parts us" explains a lot. Thanks!