While the links are difficult to read in dark mode under normal conditions, they are really impossible to read if you are in a setting with bright light (outdoors in the sun) or if you have the screen dimmed.
I was reminded of this recently when I wrote a phone number in a message to a friend. I wanted to read the number again to double check I sent the correct number, but the phone number was automatically converted to a link and with the low color contrast of blue on gray I could not read the number at all without bringing my phone very close to my face. But obviously this is not great UX! I could read all the other text in the message and the UI elements without any problem, only the phone number which displayed as a link was unreadable.
I expect that some user testing with the colors and readability of links would confirm this is a usability issue.
Steps to reproduce the problem
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Switch to dark theme
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Dim the screen or go outside in the sun
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Write a message with a link or some text which will convert to a link
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Try to read the link