In Matomo 3.13.1 the changelog states that this issue was fixed:

23 Update dependencies: Warning RedisCache.php(135): Deprecated – Function Redis::delete() is deprecated

However, the older version of: "Common/Cache/RedisCache.php" is included in the release which does not have the fix. Specifically, in the Matomo 3.13.1 build, see the file: "Common/Cache/RedisCache.php" includes "$this->redis->delete" and not the corrected code: "$this->redis->del". Perhaps an incorrect branch of "cache" was used to build Matomo 3.13.1?

matomo-org/component-cache#23