Downgrading a new Macbook Air to Mojave

[nexu]

Hey
I'm planning on buying a new Macbook Air and I'm wondering if its possible to downgrade a new Macbook Air with preinstalled Catalina to Mojave for 32 bit app support. I've heard its not that easy to downgrade macs with T2 chip due to firmware incompatibility, some disc partitions problems after the downgrade (as Catalina creates a read-only one) and that usually you can only downgrade a mac to the version it came with.
How does it look like in pactice? I don't want to use any time machine backup, just buy it, downgrade it and use it as new.

 

[barbu]

I think you would be OK if you wiped the SSD and started with the Mojave installer. This will get you around having to reverse the logical disk changes in Catalina manually.
Just make a Mojave USB disk, boot, run Disk Util to wipe the SSD and set it back up, then install. You may need to unlock the disk with the admin password, so make sure to set one first! (i.e. boot into Catalina and set up your account - even though you will be erasing it, you need a password set that the T2 can use).

At least, that is my understanding. It is also possible such a new machine will reject the Mojave installer.

 

[26139]

The rule I've always read is that you cannot install an earlier OS than the one the machine had pre-installed.