Like every anti-vax claim, this is total bullshit.
From 1980: https://www.ahajournals.org/doi/10.1161/01.CIR.62.2.218
From 2016: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4969030/
From 2015: https://www.tandfonline.com/.../10.../00913847.2015.1001306
From 2017: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5435153
A Wikipedia article on the subject predating COVID, complete with a huge list of pre-COVID incidents:
https://en.m.wikipedia.org/wiki/Sudden_cardiac_death_of_athletes
When you point these things out to anti-vaxers (who just finished claiming that it’s a new phenomenon), they make the equally spurious claim that the RATE of cardiac incidents among athletes is up.
They make this claim based on nothing, as they always do, and they support it with nothing, since no support exists.
I haven’t seen data on the rate of cardiac incidents among athletes recently, but if it is up, it’s more likely to be from COVID, which has been demonstrated to increase cardiac events significantly, than from the vaccine, which is many orders of magnitude safer.
Long story short, anti-vaxers are unserious, and largely stupid people.