Since Drupal 8, the community is actively updating the source code. Deprecations are something to deal with. This blog post shows you how to monitor your code.
There is a simple way for checking. The drupal-check package will inspect your code:
composer require mglaman/drupal-check --dev
Now run
drupal-check -a web/modules/custom
The result was the following:
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Line alter_entity_autocomplete/src/EntityAutocompleteMatcher.php
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28 Cannot call method getReferenceableEntities() on object|false.
35 Call to an undefined static method Drupal::entityManager().
36 Call to an undefined static method Drupal::entityManager().
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Now look up de the deprecation, fix it and run again! :)