T-Mobile does add software to phones. Wi-Fi calling requires T-Mobile-specific OS-level modifications. You won't be able to use it on unbranded unlocked phones. It's definitely not bloatware though.

WiFi calling is supported on iOS devices across carriers and without T-Mobile making os level modifications.

https://support.apple.com/en-us/HT204039

I'm pretty sure the Nexus 5X I got off of craigslist is an unbranded, unlocked phone with an OS from Google, and it makes wifi calls on T-Mobile.

It would not surprise me to see kind of an opposite thing - such as other carriers selling phones where they purposefully turned off or hid settings for wi-fi calling, turning on hotspot and similar.. in which case if tmobile sold the same phone, then it would be a matter of them not turning off / hiding.

Would be an interesting chart to show the things the various carriers had turned on / off... added / removed which similar phones.

I think its already a part of the Android software. I certainly see it enabled on the phone I bought outside of T-Mobile. The only thing I see I need is a the T_Mobile visual mail app but I don't use that much since everything is SMS these days.

This has been somewhat fixed in recent years as my latest unlocked Sony phone has working Wi-Fi calling on T-Mobile.