Sizin .htaccess'e eklediğiniz kodu ilk defa gördüm. Kendi sitemde de aşağıdakine benzer bir kod kullanıyor ve bahsettiğiniz uyarıyı almıyorum.
ExpiresActive On
ExpiresDefault "access plus 1 month"
ExpiresByType text/html "access plus 1 seconds"
ExpiresByType text/xml "access plus 1 seconds"
ExpiresByType text/plain "access plus 1 seconds"
ExpiresByType application/xml "access plus 1 seconds"
ExpiresByType application/json "access plus 1 seconds"
ExpiresByType application/rss+xml "access plus 1 hour"
ExpiresByType text/css "access plus 1 month"
ExpiresByType text/javascript "access plus 1 month"
ExpiresByType application/javascript "access plus 1 month"
ExpiresByType application/x-javascript "access plus 1 month"
ExpiresByType image/x-ico "access plus 1 month"
ExpiresByType image/x-icon "access plus 1 month"
ExpiresByType image/gif "access plus 1 month"
ExpiresByType image/png "access plus 1 month"
ExpiresByType image/jpe "access plus 1 month"
ExpiresByType image/jpg "access plus 1 month"
ExpiresByType image/jpeg "access plus 1 month"
ExpiresByType font/truetype "access plus 1 month"
ExpiresByType font/opentype "access plus 1 month"
ExpiresByType application/x-font-woff "access plus 1 month"
ExpiresByType video/ogg "access plus 1 month"
ExpiresByType audio/ogg "access plus 1 month"
ExpiresByType video/mp4 "access plus 1 month"
ExpiresByType video/webm "access plus 1 month"
ExpiresByType image/svg+xml "access plus 1 month"
ExpiresByType application/pdf "access plus 1 month"
ExpiresByType application/vnd.ms-fontobject "access plus 1 month"