RHEL Engineering is moving the tracking of its product development work on RHEL 6 through RHEL 9 to Red Hat Jira (issues.redhat.com). If you're a Red Hat customer, please continue to file support cases via the Red Hat customer portal. If you're not, please head to the "RHEL project" in Red Hat Jira and file new tickets here. Individual Bugzilla bugs in the statuses "NEW", "ASSIGNED", and "POST" are being migrated throughout September 2023. Bugs of Red Hat partners with an assigned Engineering Partner Manager (EPM) are migrated in late September as per pre-agreed dates. Bugs against components "kernel", "kernel-rt", and "kpatch" are only migrated if still in "NEW" or "ASSIGNED". If you cannot log in to RH Jira, please consult article #7032570. That failing, please send an e-mail to the RH Jira admins at rh-issues@redhat.com to troubleshoot your issue as a user management inquiry. The email creates a ServiceNow ticket with Red Hat. Individual Bugzilla bugs that are migrated will be moved to status "CLOSED", resolution "MIGRATED", and set with "MigratedToJIRA" in "Keywords". The link to the successor Jira issue will be found under "Links", have a little "two-footprint" icon next to it, and direct you to the "RHEL project" in Red Hat Jira (issue links are of type "https://issues.redhat.com/browse/RHEL-XXXX", where "X" is a digit). This same link will be available in a blue banner at the top of the page informing you that that bug has been migrated.

When I try a kickstart text install using the CentOS-Stream-9-20210909.0-x86_64-boot.iso ISO where the kickstart file has the --proxy setting in the url and repo sections 
I get the error
Kernel boot argument 'proxy' detected. Did you want to use 'inst.proxy' for the installer instead?
All Anaconda kernel boot arguments are now required to use the 'inst.' prefix

and the install can't be completed due to the missing proxy settings.

I suspect anaconda needs
https://github.com/rhinstaller/anaconda/commit/b88e10fe3029602b369c719506588097fcbdf80b
backporting or something similar

PR: https://github.com/rhinstaller/anaconda/pull/3749

NOTE: Not actually tested yet in Dracut.

So I've built a boot iso with the fix and can no longer see the reported error message in journal. This the fix seems to be working. :)

Tested with RHEL-9.0.0-20211209.t.18 and anaconda-34.25.0.23-1.el9 according to comment 1:

The warning should still be displayed when "proxy" is used on the kernel command line.
* Correct

Proxy server is used for the installation when "--proxy" is used for the installation source in the kickstart file.
* Correct, tested both "url" and "repo", both function properly with --proxy in kickstart

Proxy server is used when "inst.proxy=.." is used on the kernel command line.
* Correct

marking the bug as Verified: Tested

anaconda-34.25.0.23-1.el9 is included in RHEL-9.0.0-20211217.1, moving to VERIFIED.

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory (new packages: anaconda), and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHBA-2022:2326