Are /dev/{udp,tcp} standardized or available everywhere?

This is a feature of the shell and not the operating system.

So, for example,on Solaris 10 with ksh88 as the shell:

% cat < /dev/tcp/localhost/22
ksh: /dev/tcp/localhost/22: cannot open

However if we switch to bash:

% bash
bash-3.2$ cat < /dev/tcp/localhost/22
SSH-2.0-Sun_SSH_1.1.5

So bash interprets the /dev/tcp but ksh88 didn't.

On Solaris 11 with ksh93 as the shell:

% cat < /dev/tcp/localhost/22
SSH-2.0-Sun_SSH_2.2

So we can see it's very dependent on the shell in use.

To add on, from the Bash Info node:

Bash handles several filenames specially when they are used in redirections, as described in the following table. If the operating system on which Bash is running provides these special files, bash will use them; otherwise it will emulate them internally with the behavior described below.

'/dev/fd/FD' If FD is a valid integer, file descriptor FD is duplicated.

'/dev/stdin' File descriptor 0 is duplicated.

'/dev/stdout' File descriptor 1 is duplicated.

'/dev/stderr' File descriptor 2 is duplicated.

'/dev/tcp/HOST/PORT' If HOST is a valid hostname or Internet address, and PORT is an integer port number or service name, Bash attempts to open the corresponding TCP socket.

'/dev/udp/HOST/PORT' If HOST is a valid hostname or Internet address, and PORT is an integer port number or service name, Bash attempts to open the corresponding UDP socket.