The licensing situation for CP/M is both complicated and very simple. The tl;dr version is that it's available under a liberal license equivalent to the MIT license.
The long version is: Digital Research sold it to Caldera, which sold it to Lineo, which had no use for it. They eventually agreed to release it under a liberal if rather ad-hoc license:
Subject: Re: Unofficial CP/M Website/licensing of CP/M material
To: gaby@gaby.de
Date sent: Fri, 19 Oct 2001 10:36:31 -0600
Let this email represent a right to use, distribute, modify, enhance and
otherwise make available in a nonexclusive manner the CP/M technology as
part of the "Unofficial CP/M Web Site" with its maintainers, developers and
community.
I further state that as Chairman and CEO of Lineo, Inc. that I have the
right to do offer such a license.
Lineo and its affiliates, partners and employees make no warranties of any
kind with regards to this technology and its usefulness or lack thereof.
---
Bryan Sparks
CEO Lineo, Inc.
http://www.lineo.com
(from The Unofficial CP/M Web Site.)
Unfortunately, this explicitly names The Unofficial CP/M Web Site as being the only point where the software could be made available. This was clearly an oversight, but unfortunately that's what the text said.
Luckily, as of 2022-07-06, Scott Chapman managed to get in touch with Bryan Sparks and get this clarified. The relevant wording is now superceded with this:
Let this paragraph represent a right to use, distribute, modify, enhance, and
otherwise make available in a nonexclusive manner CP/M and its derivatives.
This right comes from the company, DRDOS, Inc.'s purchase of Digital Research,
the company and all assets, dating back to the mid-1990's. DRDOS, Inc. and I,
Bryan Sparks, President of DRDOS, Inc. as its representative, is the owner of
CP/M and the successor in interest of Digital Research assets.
(see the attached PDF for the full text.)