How did you find out?

Wait.

Did they install certificates in your personal private phone?

This is par for the course in BYOD regimes. On a past job, I was once given the option -- not required -- to install an app called "Good" on my phone to grant access to corporate email and messaging through my phone. I found out that "Good"'s real purpose is Mobile Device Management -- tracking and controlling phones that connect to the corporate network, including forcing software installs and updates, remote monitoring of device and app activity, and installing certificates that let corporate IT MITM your device.

I told the IT department that if they want me to receive email via cellphone, they will need to provision a corporate phone for me to use because I am not installing anything of theirs on my personal device.

I never heard about it again.

But the OP’s use of the word “private” suggests that they had both a personal and a work phone, and that even the personal one was being MDM controlled. I can’t think of a reason that this would be acceptable.

Would they let you use android? and do the same if you did?

Define “private phone”. Was this a carry device or a phone you had purchased yourself? Did you access Apple confidential information on it? Did you ever use it for work purposes?

It seems unreasonable to assume the person you are replying to does not know the definition of "private phone".

Not really. Specifically,

> they needed me to participate in testing and providing feedback on the software service they were working on

makes it seem like this is not in fact a “private phone”.