Proposal: restrain the use of environment variables.

For now, Environment variables in this form are substituted before Caddyfile parsing begins, which will cause problem as the environment variables are not valid components and can't be parsed properly before they are substituted.
You can even write something like this:

example.com {
	root * /var/www/wordpress
	encode
	php_fastcgi unix//run/php/php-version-fpm.sock
	file_server
{$CLOSEBRACKET}

Restrain the use of environment variables so it can be interpreted as one of the structural components describled here would be a good starting point, and enforcing such rule should encourage people to write better structured Caddyfile when using environment variables.

Start with the linked Caddyfile environment-variable and structure documentation, then read issue 6881 for the reported parsing problem. Determine whether this website issue should document or define a restriction, and consider the work complete when the intended rule and its scope are unambiguous to Caddyfile users.