I would guess that all comes down to author names and enumerations (user IDs) being read at one site and then searched globally at other sites whether or not they might happen to be yours. I have my own sites set for complete anonymity so authors cannot be identified even there, but that likely goes far beyond what is concerning you. In any case, here is the code I use:
## do not allow user ID enumeration
RewriteEngine On
RewriteCond %{QUERY_STRING} ^author=([0-9]){1,}$ [NC]
RewriteRule ^(.*)$ $1?author=999999 [L]
# send /author/ scans to Home Page
RedirectMatch (?i)^/author.*$ /
https://wordpress.org/plugins/search.php?q=enumeration
I don’t even know how followers from one site find my other sites. Is there any way to stop this? I want to keep everything seperate.
Don’t use Multisite.
Seriously. That’s the ONLY way to keep it all 100% separate.