Taproot is already supported on your wallet if I understand correctly or not?
We support sending to Taproot addresses starting with witness version (1). I'm not sure what current developments are (if there have been more witness versions introduced) We don't support generating taproot deposit addresses, not until bitcoin core supports it anyway.
Is there any instructions I can read before I use it, and what should be written instead of mainnet custodian url address?
double click the input field and the url for the testnet custodian should show up (
https://testnet.blindmixer.com/#pubmp1qf98kxatpw43mqjft6j72dps5wn66yzp47k3zm0yn4skhedv32x5sphklj2)
After that, simply deposit using a lightning testnet wallet or a testnet faucet.
Does that mean that the wallet cannot be restored anywhere else even with the mnemonic phrase?
Correct. Since the wallet is fully custodial it can only be restored with your current custodian.
Also, I would've understood if the URL had some kind of unique identifier in it, but it's just displaying the home page link?
This was done to reemphasize the importance of remembering the right custodian in case there would have been multiple operators running the blindmixer software, which has not been the case thus far. Still, you might get confused between testnet and mainnet wallets.