In what way “no longer seems accessible”?

It's not an option in the app.

 It’s not available on Android phones.  If your phone is an Android try to find someone with an Apple phone to let you do Trueplay.

You are wrong about that, as you can see from the screenshot below from my Android phone and a single Era 300 I have in another room. Trueplay disappeared after I connected the rear speakers to the Arc. I'm wondering if I just need to start all over again. 

 Interesting!  I was told Sonos could not do Trueplay on Android because of the microphones on Android phones vary too much.

https://support.sonos.com/en-gb/article/trueplay-compatible-devices-and-sonos-products#

 Not sure how old this is.

The Era's can Trueplay tune themselves if they are not bonded to a Sonos soundbar. This tuning can be started from an Android phone. Since the Era's cannot tune themselves when bonded to a soundbar, it seems logical this button disappears when you bond them.

The surround "room” in the app can only be Trueplayed using an iOS device.

Thanks ya'll. I borrowed an iPhone and good to go.