Seems there was a reason that Xcode13 was needed for BS as well, I don’t recall why, but I was forced to update to Xcode13 a week or two ago…
And what reason would you have to NOT upgrade? (legacy 32bit macOS is the only reason I can think of)
The Xcode 13 release coincided with that of iOS 15, etc., and provides the SDKs for those. They stripped out the Monterey SDK that is in the Xcode 13 beta.
I indicated that I needed Xcode13 for BS, not Monterey… So it makes sense that 12 is no longer supported there either…
But it sucks that this will force you (and others) to pay XOJO yet again…if you want to use their lame iOS product
macOS
I did understand that the context was Big Sur. However, I get your concern regarding Xojo.
One can have multiple versions of Xcode on a system. If needed, you can re-download 12.5.1 from the Apple developer site. In the Downloads section, go to “More” and scroll down. It will be a very large .xip file.
Create a subfolder in /Applications. Put the downloaded .xip in this subfolder and extract it.
In Terminal, you can toggle between the Xcode version in use with this:
sudo xcode-select --switch /path/to/desired/xcode
I am guessing that a major concern of yours is access to the iOS simulator. I haven’t tried that, but I hope this helps.
However, I get your concern regarding Xojo.
guess you misunderstood… because I could care less what versions of Xcode work with Xojo
The point for me was Xcode 12 did not work with iOS15 device simulators with SWIFT
I guess your mention of Xojo threw me off 
But my suggestion might help those Xojo users who can’t/won’t upgrade.
Seems there was a reason that Xcode13 was needed for BS as well, I don’t recall why, but I was forced to update to Xcode13 a week or two ago…
You answer your own question with
The point for me was Xcode 12 did not work with iOS15 device simulators with SWIFT
Hence, when iOS 15 was shipped, Xcode 13 was released for Big Sur. As I said, it is just stripped of the Monterey tools, and will likely be updated as soon as Monterey is released