Without any announcement T-Mobile appears to have dropped tethering from the Full Monty plans. If you look here Pay Monthly Phone Tariff - T-Mobile & click on any of the Full Monty (Unlimited talk, texts & data on your phone) tariffs it states 'Excluding tethering' then if you click on the link for 'Still want tethering' you are taken back to the page of Monthly plans where the only deals that allow tethering are capped at 1.5GB.
Unlike Three's One Plan it was never available on the SIM-only version but was available when you bought a handset on contract. Presumably existing Full Monty subscribers are still permitted tethering.
One of their webpages still promises tethering so anyone who take out a Full Monty contract with handset has a good chance of getting out of the contract with a free handset if they try to retrospectively change the T&Cs. T-Mobile Full Monty tariff: Unlimited Everything on T-Mobile
Unlike Three's One Plan it was never available on the SIM-only version but was available when you bought a handset on contract. Presumably existing Full Monty subscribers are still permitted tethering.
One of their webpages still promises tethering so anyone who take out a Full Monty contract with handset has a good chance of getting out of the contract with a free handset if they try to retrospectively change the T&Cs. T-Mobile Full Monty tariff: Unlimited Everything on T-Mobile