How can I insert the client's IP back to them in a HTTP:RESPOND content

Special Cases for Using Braces

You can also use braces to enclose strings, just as you would double quotes, but variables won't expand within them unless a command also inside the braces refers to them. Because of that difference, if you need to handle strings containing $, \, [], (), or other special characters, braces will work where double quotes won't.iRules Optimization 101 - 04 - Delimiters: Braces, Brackets, Quotes and more by Deb Allen

https://devcentral.f5.com/tech-tips/articles/irules-optimization-101-04-delimiters-braces-brackets-quotes-and-more

so, you may try something like this.

e.g.

 example1

[root@ve10:Active] config  b rule myrule list
rule myrule {
   when HTTP_REQUEST {
   set data "\r\nYour [IP::client_addr] has not been recognized.\r\n\r\nPlease contact company support.false\r\n"
   HTTP::respond 200 content $data noserver "Content-Type" "text/xml; charset=utf-8"
}
}

[root@centos17 ~] curl http://172.28.19.252

Your 172.28.20.17 has not been recognized.

Please contact company support.false

 example2

[root@ve10:Active] config  b rule myrule list
rule myrule {
   when RULE_INIT {
   set static::data {
Your IP address has not been recognized.

Please contact company support.false
}
}
when HTTP_REQUEST {
   HTTP::respond 200 content [string map [list "IP address" [IP::client_addr]] $static::data] noserver "Content-Type" "text/xml; charset=utf-8"
}
}

[root@centos17 ~] curl http://172.28.19.252

Your 172.28.20.17 has not been recognized.

Please contact company support.false