This issue was noted when using Khoi Hoang's NRF52_MBED_TimerInterrupt library
https://github.com/khoih-prog/NRF52_MBED_TimerInterrupt
and was reported here
https://github.com/khoih-prog/NRF52_MBED_TimerInterrupt/issues/6

It was confirmed by that in Core 1.3.2 NRF_TIMER_1 could be used to trigger an interrupt, but that functionality was broken with the transition to Core 2.0.0.

Can NRF_TIMER_1 availability be brought back?
If not, documentation needs to be changed to indicate that it is not available for timer interrupts.