There are several known problems with external link checking in Linkvalidator,
"False positives": Some links are reported broken, but are not broken, see
External sites are checked without rate limit which may cause sites which
perform link checking to be blocked, see forge#89287.
No caching of results (except for a runtime cache during link checking which
will be invalid on next run)
These issues are currently not easily solvable and should also be addressed
specifically for the site concerned.
We now deactivate checking external link types by default in the configuration:
The "external" link types checking still works but must be enabled explicitly.
This will make administrators more aware of problems and the specific problems
can be addressed, for example, by providing a custom class to replace
\TYPO3\CMS\Linkvalidator\Linktype\ExternalLinktype. Additionally, a page
Known Problems was
already added to the documentation in a previous patch.
Either leave external link checking deactivated or find ways to mitigate the
problems with external link checking.
More information is available in the Linkvalidator documentation:
Example for activating external linktype
mod.linkvalidator.linktypes = db,file,external