I am really amazed that Magento pushed this to release without documenting it in the release note or "backwards incompatible" changes, but they did. They mention it in the installation notes, as pointed out in Diana's answer, but nowhere else.
There are several bugs filed on github where people share workarounds they are trying. Main one:
https://jokiruiz.com/magento-2/how-to-run-magento-2-from-a-subdirectory/
and obviously the other options is making the change prior to upgrade https://devdocs.magento.com/guides/v2.4/install-gde/tutorials/change-docroot-to-pub.html
attempt #1 subfolder (not visible in url)
For testing purposes I have a test site upgraded to 2.4.2 and managed to find a config that uses a subfolder BUT the subfolder is invisible in the url.
- magento installed in magento2-path subfolder
- site working on www.mywebsite.com
- if magento2-path was visible in url, and therefore set up in the site url-path config or in stylesheets, that must be cleaned out
- www.mywebsite.com/magento2-path will 404
Note 2: to save work this does not have a local copy of the images and uses "static domain" for images so not certain it 100% correct for static files and probably some extra rule needed for pub/static
site-root
.htaccess
magento2-path
.htaccess
pub
.htaccess
the htaccess at the top has
RewriteEngine on
RewriteCond %{REQUEST_URI} !^/pub/
RewriteCond %{REQUEST_URI} !^/setup/
RewriteCond %{REQUEST_URI} !^/update/
RewriteCond %{REQUEST_URI} !^/dev/
RewriteCond %{REQUEST_URI} !^/magento2-path/setup/
RewriteCond %{REQUEST_URI} !^/magento2-path/update/
RewriteCond %{REQUEST_URI} !^/magento2-path/dev/
RewriteCond %{REQUEST_URI} !^/magento2-path/pub/
RewriteCond %{REQUEST_URI} !^/magento2-path/
RewriteRule .* /magento2-path/pub/$0 [L]
RewriteCond %{REQUEST_URI} ^/pub/
RewriteRule .* /magento2-path/$0 [L]
DirectoryIndex index.php
the htaccess in magento2-path is modified to (the / in front of pub is removed)
RewriteEngine on
RewriteCond %{REQUEST_URI} !^pub/
RewriteCond %{REQUEST_URI} !^/setup/
RewriteCond %{REQUEST_URI} !^/update/
RewriteCond %{REQUEST_URI} !^/dev/
RewriteRule .* pub/$0 [L]
DirectoryIndex index.php
the htaccess in pub is not modified, except to add any special old "migration" rules I had from migration from past modules or pages that are now gone
attempt #2 subfolder (with redirect)
UPDATE: I have found that for a site that used the magento install path in the URL I can make it redirect so the page shows (without that path) only when the folder is renamed to not match the path anymore. Otherwise requests get passed down the stack. You want to make sure your "root level" rewrites are used, not the magento one.
magento was installed in magento2-path subfolder RENAME THIS FOLDER eg tomagento2-folder
site working on www.mywebsite.com
if magento2-path was visible in url, and therefore set up in the site url-path config or in stylesheets, that must be cleaned out
www.mywebsite.com/magento2-path/whatever redirects to www.mywebsite.com/whatever which displays correctly
site-root
.htaccess
magento2-folder2
.htaccess
pub
.htaccess
the htaccess at the top has (note: this is total overkill i'm sure only half the redirects are needed but not got time to clean)
RewriteEngine on
RewriteRule ^/magento2-path/$ / [R=301,L]
RewriteRule ^/magento2-path$ / [R=301,L]
RewriteRule ^magento2-path/$ / [R=301,L]
RewriteRule ^magento2-path$ / [R=301,L]
RewriteCond %{REQUEST_URI} /magento2-path/
RewriteCond %{REQUEST_URI} !^/magento2-path/setup/
RewriteCond %{REQUEST_URI} !^/magento2-path/update/
RewriteCond %{REQUEST_URI} !^/magento2-path/dev/
RewriteCond %{REQUEST_URI} !^/magento2-path/pub/
RewriteRule ^magento2-path(.*) $1 [R=301,L]
RewriteCond %{REQUEST_URI} /alpineshop/
RewriteCond %{REQUEST_URI} !^/magento2-path/setup/
RewriteCond %{REQUEST_URI} !^/magento2-path/update/
RewriteCond %{REQUEST_URI} !^/magento2-path/dev/
RewriteCond %{REQUEST_URI} !^/magento2-path/pub/
RewriteRule ^/magento2-path/(.*) /$1 [R=301,L]
RewriteCond %{REQUEST_URI} !^/pub/
RewriteCond %{REQUEST_URI} !^/setup/
RewriteCond %{REQUEST_URI} !^/update/
RewriteCond %{REQUEST_URI} !^/dev/
RewriteCond %{REQUEST_URI} !^/magento2-path/setup/
RewriteCond %{REQUEST_URI} !^/magento2-path/update/
RewriteCond %{REQUEST_URI} !^/magento2-path/dev/
RewriteCond %{REQUEST_URI} !^/magento2-path/
RewriteRule .* /magento2-folder/pub/$0 [L]
RewriteCond %{REQUEST_URI} ^/pub/
RewriteRule .* /magento2-path/$0 [L]
DirectoryIndex index.php
the htaccess in magento2-folder and magento2-folder/pub is the same as above
