TUPE protections
A. There’s no time limit on the protection provided by the Transfer of Undertakings (Protection of Employment) Regulations 2006 (TUPE), except in relation to collectively agreed terms over transfers that took place on or after 31 January 2014.
In all other cases, Reg 4 of TUPE 2006 (as amended) applies. This makes a variation of contract void if the “sole or principal reason for the variation is the transfer”, unless that reason is an economic, technical, or organisational (ETO) reason entailing changes in the workforce.
Since there is no suggestion of any changes to the workforce, either in the number of employees or your job functions, there can be no ETO reason, so the only question is whether the reason for the removal of the leave is the transfer.
There is no obvious link between the change to holiday rights and the transfer, especially since 10 years have passed. But it would be worth pushing your employer for an explanation so that you can examine this further.