Well, not only this data, all activity on lemmy is public since it needs to be federated (sent to all instances subscribed to the community will receive all activity).
Which means any person can track anyone if they subscribe to the same communities the user’s instance has.

AFAIK the only activity not sent is saved content, and downvotes from content hosted in instances which disabled them.

EDIT: for more example, here’s my upvote to this post

"actor":"https://lemmy.pe1uca.dev/u/pe1uca","object":"https://sh.itjust.works/post/8931097","type":"Like","id":"https://lemmy.pe1uca.dev/activities/like/f6b0cced-4e1c-41d7-bf11-349b680c4d84","audience":"https://lemmy.one/c/privacyguides"  

And here’s the original comment

actor":"https://lemmy.pe1uca.dev/u/pe1uca","to":["https://www.w3.org/ns/activitystreams#Public"],"object":{"type":"Note","id":"https://lemmy.pe1uca.dev/comment/1434121","attributedTo":"https://lemmy.pe1uca.dev/u/pe1uca","to":["https://www.w3.org/ns/activitystreams#Public"],"cc":["https://lemmy.one/c/privacyguides","https://sh.itjust.works/u/andrew_bidlaw"],"content":"","mediaType":"text/markdown"},"published":"2023-11-11T04:07:31.962497+00:00","tag":[{"href":"https://sh.itjust.works/u/andrew_bidlaw","name":"@andrew_bidlaw@sh.itjust.works","type":"Mention"}],"distinguished":false,"language":{"identifier":"en","name":"English"},"audience":"https://lemmy.one/c/privacyguides"},"cc":["https://lemmy.one/c/privacyguides","https://sh.itjust.works/u/andrew_bidlaw"],"tag":[{"href":"https://sh.itjust.works/u/andrew_bidlaw","name":"@andrew_bidlaw@sh.itjust.works","type":"Mention"}],"type":"Create","id":"https://lemmy.pe1uca.dev/activities/create/7a1c726e-0191-4a71-8980-a565727ac52d","audience":"https://lemmy.one/c/privacyguides"   

And all instances which are subscribed to this community need to receive this information to keep it updated.