Is it legal to use OpenCore Legacy Patcher to install macOS?

Legal, yes. Whether it would be wise is up to you. If you're willing to deal with the various security compromises, probable incompatibilities and likely other headaches, then you can try it. Be prepared to have to erase your Mac and go back to the release version of macOS, though.

Regards.

OCLP on its own may be legal software. But using it on an Apple Mac to modify the operating system in order to run a version of macOS that is not qualified for the specific Mac model in question may put you in violation of the license for macOS. Plus, you will be on your own; Apple will not provide support; and users in this community will also probably not provide assistance when you inevitably run into problems,.

I believe the relevant section of the Apple Software License Agreement is this:

N. No Reverse Engineering. You may not, and you agree not to or enable others to, copy (except as

expressly permitted by this License or by the Usage Rules if they are applicable to you), decompile,

reverse engineer, disassemble, attempt to derive the source code of, decrypt, modify, or create

derivative works of the Apple Software or any services provided by the Apple Software or any part

thereof (except as and only to the extent any foregoing restriction is prohibited by applicable law or by

licensing terms governing use of Open-Sourced Components that may be included with the Apple

Software).

(Emphasis mine. Note, this is from the Sequoia license but the same provision is in previous versions of the license)

Also note, that more often than not, whenever there is a macOS update, even minor, you will need to go through the entire process yet again.

And as previously mentioned there can be many other pitfalls of running an unsupported version of macOS and not just with the OS itself but apps as well which many rely on the supported OS and the supported hardware that it runs on.

In general, it can be risky and you better have a very robust multiple backup scheme.

Reverse engineering the Apple hardware or any operating system by any means would be a violation of the stated Apple licensing agreements. Apple would know that you are running a version of macOS on unsupported hardware.

Then, Apple legal will decide to reach out and touch you, or simply disable the operating system. It would be in their legal right to do so as you have violated Apple's terms of usage for the operating system that you implicitly agreed to when you installed it.

You certainly wave your right to any repair by Apple or Apple Authorized Service centers, and not unlike beta software, anyone in these support communities, since we choose not to be complicit in the violation of Apple's licensing terms.