`Family.cross_projects` was implemented in compat for login purpose but never used in core. I guess it is not necessary to have it:
```
de = pywikibot.Site('de')
de
APISite('de', 'wikipedia')
en = pywikibot.Site('en')
en
APISite('en', 'wikipedia')
de.logged_in()
False
en.logged_in()
False
de.login()
Password for user <redacted> on wikipedia:de (no characters will be shown): Warning: Password input may be echoed.
<redacted
WARNING: C:\Python\Python313\Lib\getpass.py:99: GetPassWarning: Can not control echo on the terminal.
return fallback_getpass(prompt, stream)
Du musst deinen Anmeldeversuch verifizieren. Ein Verifizierungscode wurde an
deine E-Mail-Adresse (***@***.***) versandt. Bitte gib hier diesen Code ein.
Verifizierungscode: Warning: Password input may be echoed.
<redacted>
de.logged_in()
True
en.logged_in()
False
en.login()
en.logged_in()
True
```
Therefore I propose to deprecate it.