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I regularly bookmark YouTube links and Safari never crashed. Now I'm on Sur 11.6.5 with Safari 15.3 (16612.4.9.1.8, 16612).
You probably have an extension, like AdGuard, that’s causing this problem.
Another possible cause could be iCloud Bookmarks. Try to temporary disable it, if you use it.
 
I regularly bookmark YouTube links and Safari never crashed. Now I'm on Sur 11.6.5 with Safari 15.3 (16612.4.9.1.8, 16612).
You probably have an extension, like AdGuard, that’s causing this problem.
Another possible cause could be iCloud Bookmarks. Try to temporary disable it, if you use it.

I upgraded to the beta of 15.4 now, seems to have been resolved, very weird..
 
Safari 15.4 has been released just now

Code:
http://swcdn.apple.com/content/downloads/27/58/002-81938-A_JGLR6R7D8U/kfpfiwbr575oxwvs6quv4oova5gu6l6y0c/Safari15.4BigSurAuto.pkg
 
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