When CGBetterForms sends mail with PHP 8, most headers are indented by a single space, rendering them disabled. Broken headers include: From, Reply-To, Message-ID, X-Mailer, MIME-Version and Content-Type.
Not sure if this a module issue or an issue with core?
CGBetterForms & PHP 8 problem
Re: CGBetterForms & PHP 8 problem
PHP 8 won't have full core support for a while, I'm pretty impressed you were able to get as far as you did.
Not getting the answer you need? CMSMS support options
Re: CGBetterForms & PHP 8 problem
Happy to report PHP 8 runs CMSMS perfectly with the modules I use, this was the only issue I encountered.