Thanks for the follow-up and additional info.
Based on your requirements, you may leverage Azure App service Linux custom container rather than Azure App Service built-in Linux (not feasible option for SSI).
NGINX Rewrite Rules for Azure App Service Linux PHP 8.x
PHP version 8.x uses NGINX as the default configuration file on Azure app services Linux and the default location of this file on server is - /etc/nginx/sites-available/default.
Just to add more info:
You may make customized Nginx configuration based on the default one, changes to the /home/default file. Set up a custom startup script and modify the existing NGINX site configuration.
In the Azure Portal > Navigate to your WebApp > Go to `Configuration >> General settings >> Add a Startup Command”
I have also relayed your feedback internally to our product team.