Tell someone already put the latest firmware update on their wallets: one, t version? From the changes they write that only 1 cryptocurrency Bitcoin is available and there is an instruction on how to correctly set everything up. The wallet cease to be multi currency or is it a special firmware for adherents of bitcoin?
Bitcoin-only firmware is for people who don't use any other crypto or believe that their implementations might include security holes. It is not mandatory. The wallet remains to support multiple cryptocurrencies if one doesn't update to the special version of the firmware. I have updated my Trezor T to the latest version and I can confirm that.
I can also confirm BitCryptex' observation. I have a couple of altcoins stored on my Trezor T which I recently updated to the latest firmware. My altcoin accounts are still accessible and I've experienced no issues.
From what I've seen the bitcoin-only firmware must be installed manually, in other words you would know you're installing a non-standard version.