How to delete directory ~/Pictures? This is on MacOS Catalina, but the same behavior has been observed in prior versions of MacOS.
Similar questions have been asked for other default directories in the default home directory, but this is different. I am able to delete most of the other directories: ~/Music, ~/Public, and ~/Movies all went away after I removed any ACL's from them. But ~/Pictures is different, and no matter what I do, MacOS refuses to delete it.
bash-5.0# /bin/ls -led Pictures
drwxrwxr-x  2 MyLogin  MyGroup  64 Nov  5 18:17 Pictures
bash-5.0# /bin/ls -lea Pictures
total 0
drwxrwxr-x   2 MyLogin  MyGroup    64 Nov  5 18:17 .
drwxr-xr-x+ 72 MyLogin  MyGroup  2304 Nov  5 18:17 ..
 0: group:everyone deny delete

Note that the last ACL is applied to the parent directory of ~/Pictures ie. my home directory, and that is not the cause of this behavior as I am able to delete other subdirectories without trouble.
This also does not appear to be a restriction imposed by System Integrity Protection:
bash-5.0# /bin/ls -leOd /usr/bin
drwxr-xr-x  1014 root  wheel  restricted 32448 Oct 23 01:19 /usr/bin
bash-5.0# /bin/ls -leOd Pictures/
drwxrwxr-x  2 MyLogin  MyGroup  - 64 Nov  5 18:23 Pictures/
bash-5.0# /bin/ls -leOa Pictures/
total 0
drwxrwxr-x   2 MyLogin  MyGroup  -   64 Nov  5 18:23 .
drwxr-xr-x+ 72 MyLogin  MyGroup  - 2304 Nov  5 18:23 ..
 0: group:everyone deny delete

Note the restricted attribute on /usr/bin which is protected by SIP. ~/Pictures has no such attribute.
I've tried as root, as my normal user account, and even as a different user after setting the permission 777. Nothing works.
My question is: what is protecting ~/Pictures from deletion, and how can I turn it off?
My question is: what is protecting ~/Pictures from deletion, and how can I turn it off?

In macOS Catalina the Users &gt; $USER &gt; Pictures folder, aside from ACL's set on it, it is being protected by System Integrity Protection (SIP).
Even using sudo chmod -N Pictures, while in the Home directory in Terminal, the directory can not be deleted unless done from macOS Recovery using Terminal without turning off SIP.

Caution: This will remove the Pictures directory and everything in it!

Example:
rm -r &quot;/Volumes/Macintosh HD - Data/Users/name/Pictures&quot;


Replacing name in /name/ with your short name, the name of your Home directory.
Replacing Macintosh HD as appropriate if you've modified the default name.

If you want to be able to delete if from a normal running occurrence of macOS Catalina, not macOS Recovery, then you will need to turn off SIP from macOS Recovery using Terminal from the Utilities menu.
csrutil disable

You could then immediately use:
csrutil enable --without fs

This way, except for the filesystem, the other protections offered by SIP are still in play.
Personally, I do not recommend doing this and would opt for deleting it from macOS Recovery without turning off SIP and then once booted back to normal mode macOS Catalina... I'd create the directory from Terminal while setting it hidden with SetFile, applying ACL's to prevent any modifications while also removing all normal privileges (rwx), all in the hope that Apple doesn't mess with it until the next upgrade.
That said, if your are the type that always shows all files including the hidden ones, then what's mentioned in the paragraph above is a moot point.

Notes:
To boot to macOS Recovery from a running occurrence, normal mode boot, of macOS, you can use the following compound command from Terminal:
sudo nvram &quot;recovery-boot-mode=unused&quot;; sleep 2; sudo reboot

Then before rebooting from macOS Recovery, in Terminal from the Utilities menu, run:
nvram -d recovery-boot-mode

The latter is not necessary in some versions of macOS, however, it doesn't hurt either way.