A warning about future breakage.
WHAT IT MEANS
The code uses something that still works and will be removed in a future PHP version.
Not an error. A warning about what will break later.
WHY IT MATTERS
Deprecation notices in your current logs predict fatal errors when you upgrade PHP.
Reading them before upgrading is considerably better than discovering them afterwards.
WHAT TO DO
If they come from a plugin or theme, check for an update. Maintained components fix these.
If a component has not been updated to address them, that is a signal it may be abandoned.
If they come from your own code, update it.
WHY THEY SUDDENLY APPEAR
A PHP version change. Moving to a newer version surfaces deprecations that the older one did not report.
HIDING THEM
Set error display off on a live site, which you should do anyway. They still log.
Do not suppress the logging; those notices are useful.
BEFORE UPGRADING PHP
Read them. They tell you exactly what will break.
