Hi Trin,
I have the feeling that you might be posted this into the wrong place: you can ask questions in the support forum where I do my best to give you the best answers I can give you.
Based on your message you checked the Google Tag Assistant chrome extension.
First of all: Tag Assistant can sometimes give you false positive results but non-standard implementation is also shown in many other cases. Many professionals use Google Tas Assistant less and less frequent as it can give you more headache than answers.
The ‘non-standard implementation’ message is marked with the blue badge. The official help center article says this about any blue badge:
A blue indicator will show there are suggestions on improving overall tagging health
Which is nothing more but a suggestion. If you would see a yellow or red badge, that would mean some issues that you need to take care for sure.
But anyway, let me give you some more details about why you might see a non-standard implementation message in Google Tag Assistant using GTM4WP:
#1 outdated WordPress theme
When GTM4WP was created, WordPress had no offical option to place codes after the opening body tag in your page which is the correct place for the so called noscript part of the GTM container code.
During the years, different theme creators added different methods to achieve this. Only WordPress 5.2 introduced a standard way to add codes after the opening body tag but this still has to be adapted by themes which might be a slower process
This is why there are currently multiple container placement options in GTM4WP:
a) the default selection is “footer”: this is not the standard way but it compatible with most WordPress instances and also technically working fine but Tag Assistant will mark this as non-standard
b) you have an option called “codeless”: this will pick up many custom solutions found in themes and theme builders and also support the standard way introduced in WordPress 5.2. But if this would be the default option, I would get many 1* reviews because “this plugin does not work, it can not place a simple line of code into my site, what a …” you can guess the rest of the message ??
c) for more advanced or let’s say more complicated cases, there is manual option which can be used with themes where option #2 is not working and you do you want to use option #1 which is obviously a non-standard implementation
d) and there is the “off” option which allows you to fully manually add your GTM container code but is this rarely used in very edge cases
#2 additional codes in the main GTM container code added by GTM4WP
During the years, I’ve got many support request since WordPress is a big environment and there are many combinations of plugins and themes where GTM4WP needs to be operating correctly.
One case is when users use Cloudflare Rocket Loader for example. Thanks to the help of other plugin users, it turned out that there is a quite simple solution for solving compatibility issues. Adding this to the GTM container code made many plugin users very happy though this also resulted Google Tag Assistant to show a blue badge with the non-standard implementation message. Technically the container code is still working perfectly. But due to the added HTML attribute, it considers the implementation non-standard which is obviously true ??
Developing a plugin is quite hard, at the beginning you only have a simple task:
make a plugin that can place two code snippets in the correct location
But as you have more and more users, you will realize that the task is more complex as the WordPress economy is more complex.
At the end of the day, you might need to decide from only two options: having 1* reviews from users actually not able to use the plugin or having 1* reviews from users where the plugin might actually work but a tool shows an innocent message ??
Again: I am happy to have a look at your site and check whether GTM is loading correctly besides this message. But as you might now see: the non-standard implementation message does not necessarily mean that your GTM container is not loading properly.
