My understanding has always been that building iOS apps on other OSes is more of a legal/licensing challenge than a technical one. For example, quoting from the Apple Developer Program License Agreement:
2.1 Permitted Uses and Restrictions; Program Services
Subject to the terms and conditions of this Agreement, Apple hereby grants You during the Term, a limited, non-exclusive, personal, revocable, non-sublicensable and non-transferable license to:
- Install a reasonable number of copies of the Apple Software provided to You under the Program on Apple-branded products owned or controlled by You, to be used internally by You or Your Authorized Developers for the sole purpose of developing or testing Covered Products designed to operate on the applicable Apple-branded products, except as otherwise expressly permitted in this Agreement;
2.6 No Other Permitted Uses
Except as otherwise set forth in this Agreement, You agree not to rent, lease, lend, upload to or host on any website or server, sell, redistribute, or sublicense the Apple Software, Apple Certificates, or any Services, in whole or in part, or to enable others to do so. You may not use the Apple Software, Apple Certificates, or any Services provided hereunder for any purpose not expressly permitted by this Agreement, including any applicable Attachments and Schedules. You agree not to install, use or run the Apple SDKs on any non-Apple-branded computer, and not to install, use or run iOS, iPadOS, macOS, tvOS, visionOS, watchOS, and Provisioning Profiles on or in connection with devices other than Apple-branded products, or to enable others to do so. You may not and You agree not to, or to enable others to, copy (except as expressly permitted under this Agreement), decompile, reverse engineer, disassemble, attempt to derive the source code of, modify, decrypt, or create derivative works of the Apple Software, Apple Certificates or any Services provided by the Apple Software or otherwise provided hereunder, or any part thereof (except as and only to the extent any foregoing restriction is prohibited by applicable law or to the extent as may be permitted by licensing terms governing use of open-sourced components or sample code included with the Apple Software).
Now, I am not a lawyer, so I can't give a very informed opinion about whether this falls within the permitted use or whether these terms are even enforceable. I am aware that the copyright situation with regard to reverse engineering for compatibility has changed recently (see Oracle vs Google), but I'm not sure to what extent that affects the terms in the license agreement.
At the very least, I can say that it seems risky, and certainly if you do deploy directly to the AppStore (as it is suggested is coming) I would not be surprised if you were found to be in violation of the agreement. Again, not a lawyer, but my understanding of the Epic Games vs Apple case is that Apple has pretty wide latitude to terminate developer accounts.
Can you comment on this? I assume you know better than I do. What's your understanding of the licensing issue?