Hm, this is not a change in PHP 8.1, is it?

Can confirm in 3.28.9 / PHP 7.4

Confirm and debin/linux, nginx+mariadb

I can't speak for previous editions of PHP but in 7.4 the configuration file is similar:

/etc/php/7.4/apache2

; Maximum size of POST data that PHP will accept.
; Its value may be 0 to disable the limit. It is ignored if POST data reading
; is disabled through enable_post_data_reading.
; http://php.net/post-max-size
post_max_size = 0

The function getMaximumFileUploadSize() is used in about 25 places in the code.
Sometimes in the context of displaying the max file size to upload, some others to validate the filesize uploaded.

Patching it could take some effort and also in terms of testing.

So, not the greatest solution, but if the PHP setting is on 0 on PHP, can we derive it then from a config setting which coud be really big?
Having this setting on Zero is far away from usual.

Thoughts @ollehar?