A few months ago someone ran a simple stylometry tool across Hacker News comments, and identified many alts, including one for the site's founder. To further demonstrate that this isn't just an academic consideration, I recently did the same for LessWrong and the EA Forum. I'm not going to share the code or the probabilities it generated, and I've only looked at the output enough to be pretty sure it's working. Trained on half of the data and tested on the other half it was consistently able to link accounts, however, and it was also able to identify at least one non-public alt account I already knew about.

This is an example of a general problem with privacy: even if something seems careful enough now, you can't trust the future to keep things private.

(If you do want somewhat more protection now, however, I think best practice is running your alt comments through an LLM to change the style.)